Electronics Merit Badge Class Handout #2

Transcription

1 Electronics Merit Badge Class Handout #2 Counselor: Steve Willoughby Troop 547, Sunset Trail District, Cascade Pacific Council Phone: ; October 8, 2009 This handout will help you recall the topics we discussed in class, and to go on from there as you explore these topics more on your own. There are several example circuits presented in this handout, all of which would be acceptable for use as a project to satisfy requirement #4. There is a lot of information in this handout. You are not expected to know everything written here this is intended to help you remember what we discussed in class and give you additional information to help get you started exploring more in depth about electronics later, too. When you see a boxed paragraph like this, it points out what you need to know in order to qualify for the merit badge. Make sure you have a thorough understanding of that part of this document. Everything else is provided to help show you the bigger picture of how all these basic elements fit together, and to help you dive more deeply into any topic that catches your interest.

2 Electronics Merit Badge Handout #2 Colophon and Acknowledgements Text and schematic diagrams typeset using TEX, supplemented with pic, circuit macros, gnuplot, and the m4 macro processor, using the Computer Modern typeface family at 11 pt. All photographs, illustrations, and content are the original work of the author. Thanks to my friends who helped proofread this document and offered feedback on it and the accompanying class presentation: Glenn Bailey, Rusty Copeland, Jerry Elkins, Beth Gordon, Zane Healy, Brian Landers, Michael Poe, Steven Richman, Cassandra Scaggs, and Darrell Smith. Copyright c 2009 Steven L. Willoughby, Aloha, Oregon, USA. All Rights Reserved. Permission is hereby granted to reproduce these materials for non-profit, non-commercial use in teaching or studying for the Electronics Merit Badge, amateur radio, or an equivalent basic electronics class, under the terms and conditions of the Creative Commons Attribution-Noncommercial-No Derivitive Works 3.0 License. (For details on the licensing terms, see 2 [1.3.1; 07-OCT-2009]

3 CONTENTS Electronics Merit Badge Handout #2 Contents 1 Audio Circuits Sound Waves Speakers Microphones Oscillators Free-running (Astable) Multivibrator Using the 555 Chip One-shot (Monostable) Multivibrator Flip-flop (Bistable Multivibrator) Amplifiers Transistors Operational Amplifiers (Op-Amps) Example Circuit: Morse Code Practice Oscillator Control Circuits Sensors Manual Input Impact Sensors Light Sensors Motion and Proximity Sensors Temperature Sensors Relays Example Circuit: Light-Sensitive Relay Digital Circuits Binary Numbers Number Base Basics Same Tune, Different Verse (Octal) Base 16 (Hexadecimal) Binary (Base 2) Converting From Base Decimal to Octal Decimal to Hexadecimal Decimal to Binary Binary Logic YES Gates (Buffers) NOT Gates (Inverters) OR Gates [1.3.1; 07-OCT-2009]

4 Electronics Merit Badge Handout #2 CONTENTS AND Gates NOR and NAND Gates XOR (Exclusive-OR) Gates Binary Manipulation Circuits Counters Decoders Digital I/O (Input/Output) Switches Discrete LEDs Segment LEDs Example Circuit: LED Sequencer Example: Binary Counter Example: Decimal Counter Example: Free-Running Decimal Counter Test Equipment Signal Generator Logic Probe Breakout Box Logic Analyzer Handy References Powers of 2, 8, 10, and Base Conversion Table Assignments For Next Class 58 4 [1.3.1; 07-OCT-2009]

5 LIST OF FIGURES Electronics Merit Badge Handout #2 List of Figures 1 Sine Wave Square Wave Triangle Wave Sawtooth Wave Timing Diagram for Astable Multivibrator (Square-Wave) Astable Multivibrator (Oscillator) Circuit Schematic Astable Multivibrator (Oscillator) Circuit with Speaker in Astable Oscillator Mode Monostable Multivibrator (One-Shot) Circuit Schematic in Monostable Multivibrator Mode Bistable Multivibrator (Flip-Flop) Circuit Schematic Transistor Amplifying an Audio Signal Example Circuit: Morse Code Practice Oscillator Example Circuit: Morse Code Oscillator with Tuning Control Example Circuit: Morse Code Oscillator with Volume Control Schematic Symbol and Photo of Photodiode Schematic Symbol and Photo of Phototransistor Schematic Symbol and Photo of Relay Example Circuit: Light-Sensitive Relay Chip Pinout Diagram: 4050 Hex Buffer Chip Pinout Diagram: 4049 Hex Inverter Example of OR Gate Circuit Chip Pinout Diagram: 4071 Quad 2-input OR Gate Example of OR Gate Circuit with Chip Connections Chip Pinout Diagram: 4081 Quad 2-Input AND Gate Chip Pinout Diagram: 4073 Triple 3-Input AND Gate Chip Pinout Diagram: 4001 Quad 2-Input NOR Gate Chip Pinout Diagram: 4011 Quad 2-Input NAND Gate Chip Pinout Diagram: 4070 Quad 2-Input XOR Gate Chip Pinout Diagram: 4077 Quad 2-Input XNOR Gate Schematic Symbol for 4510 Decade Counter Chip Schematic Symbol for the 4028 BCD-to-Decimal Decoder Simple Pushbutton Circuit with Pull-up Resistor Simple Debounced Pushbutton Circuit Photo of 7-Segment LED Display (Single Digit) Segment LED Schematic Symbol for 4511 BCD-to-7-Segment LED Driver Example Circuit: LED Sequencer [1.3.1; 07-OCT-2009]

6 Electronics Merit Badge Handout #2 LIST OF TABLES 39 Example Circuit: Binary LED Counter Example Circuit: Decimal Counter with Digital Output Example Circuit: Free-Running Counter Modification Logic Probe Breakout Box List of Tables 1 Binary Bit Patterns for Binary Encoding of ASCII Symbols Truth Table for A (Identity Function or YES Gates ) Truth Table for A (NOT A) Decision Table for Going Back to Sleep Truth Table for A + B (A OR B) Truth Table for ABC (A AND B AND C) Truth Table for NOR and NAND Truth Table for A B C (A XOR B XOR C) and Its Inverse Truth Table for 4510 Decade Counter Chip Truth Table for 4028 Decoder Chip Table of Powers of 2, 8, 10, and Base 10, 2, 8, and 16 Conversion Table [1.3.1; 07-OCT-2009]

7 1 AUDIO CIRCUITS Electronics Merit Badge Handout #2 1 Audio Circuits To pass requirement #4(c), you need to describe three applications of electronics for audio. You should be generally familiar with how electronics is used. For example, you should know in simple terms what an amplifier is, but don t need to be able to go into the detail you ll see below. If you wish to build an audio circuit for your requirement #4 project, you may wish to read this section fully so you can be able to describe how your project works. Many of the most familiar uses for electronic circuits in our lives revolve around audio applications. We enjoy listening to our radio, television or portable MP3 player, we talk with someone on the telephone, or a dozen similar activities every day, which would not be possible (or at least vastly different) without audio circuitry. The two specific types of audio circuits we ll look at here are those which produce a sound of their own, and those which take an audio signal and amplify it. 1.1 Sound Waves Remember that sound is what we hear when something (for example, the string of an instrument) vibrates at a certain frequency. This in turn causes the air around it to vibrate, making waves of sound radiate out in all directions from the sound source. When those waves of moving air reach your eardrum, they make it vibrate in the same pattern, which your brain senses as sound. 1 If you were to graph the movement of the surface of a sound source, you would see a repeating waveform like Figure 1. This is a sine wave and sounds like a nice, smooth steady tone. The higher the frequency of the vibration (the more cycles back and forth it makes per second), the higher the pitch of the sound. In the above illustration there are several waveforms, with the lowest frequency wave on the top and the highest on the bottom. The human ear can detect sounds from about 15 Hz up to 20 khz. The shape of the waveform affects what it sounds like. Besides sine waves, we can listen to other simple waveforms such as square waves (Figure 2), triangle waves (Figure 3), and sawtooth waves (Figure 4). 1 Contrary to what many science fiction shows and movies depict, since there is no air in outer space, it is utterly silent. Sound cannot travel by itself through a vacuum. 7 [1.3.1; 07-OCT-2009]

8 Electronics Merit Badge Handout #2 1.1 Sound Waves Figure 1: Sine Wave Figure 2: Square Wave 8 [1.3.1; 07-OCT-2009]

9 1 AUDIO CIRCUITS Electronics Merit Badge Handout #2 Figure 3: Triangle Wave Figure 4: Sawtooth Wave 9 [1.3.1; 07-OCT-2009]

10 Electronics Merit Badge Handout #2 1.1 Sound Waves These sound more artificial than the sine wave. By contrast, real world sounds have much more complex patterns in their waveforms. Whatever the waveform looks like, we can use electronics to convert its vibration pattern directly into an AC electrical signal which would look identical to the graphs we just saw, if you graphed the changing voltage over time Speakers If we have an electrical waveform, whether produced by the circuit itself or just being processed by it, we can convert it to real sound waves using a speaker. The speaker, whose schematic symbol is shown on the right, consists of a thin paper cone attached to an electromagnet (which is called a voice coil ). As AC current passes through the coil of the electromagnet, it is converted to a magnetic field. This field is fluctuating in exactly the same pattern as the current. As the signal goes more positive, the magnetic field pushes the electromagnet farther away from the speaker s permanent magnet. Since the paper cone is attached to the electromagnet, it moves along with it. As the signal goes more negative, the magnetic field pulls the cone in the opposite direction. This makes the paper cone vibrate, which pushes the air around it, sending out sound waves Microphones If we are processing a sound already being made by something in our environment, we can use a component such as a microphone to convert the vibrations into an electrical signal. A microphone, whose schematic symbol is shown to the right, consists of a small, sensitive membrane (similar in operation to your eardrum) which will move very slightly back and forth as it is pushed by sound waves traveling to it through the air. This membrane is connected to a small coil of wire and a magnet so that as the sound vibration moves the coil back and forth across the magnet s field, small currents of electricity are induced in the wire. Essentially, this is the same thing as a speaker, only working in reverse. (Actually, you can use a speaker as a microphone, or vice versa, but the sound quality won t be as good since speakers and microphones are specially manufactured to be best at their own intended task, not both.) 10 [1.3.1; 07-OCT-2009]

11 1 AUDIO CIRCUITS Electronics Merit Badge Handout #2 1.2 Oscillators An oscillator is a circuit which produces a regular AC signal on its own. When we construct the circuit, we can design it so that it vibrates at whatever frequency we desire. These circuits are useful for many kinds of applications, since having a source of a regular timing pulse or timed delay is a generally useful tool. In the case of audio circuits, we will set up oscillators which send out pulses at frequencies within the range of human hearing. If we feed this signal to a speaker, we should be able to hear an audible tone Free-running (Astable) Multivibrator Let s start with the desired output we want this circuit to produce. We want to set up a vibration of back-and-forth air movement, so we need to send a back-and-forth alternating (or AC) electrical signal into a speaker. T 1 T 2 Figure 5: Timing Diagram for Astable Multivibrator (Square-Wave) To keep things simple, we ll just produce a square wave, which means that we need our circuit to turn on and off, over and over, hundreds of times a second. The waveform would look like the one in Figure 5. (The shorter the time it takes to go through one cycle (T 1 + T 2 ), the more cycles we ll have per second, and the higher the pitch will sound.) The easiest way to produce this is to build what is known as an astable multivibrator. The simplest definition of a multivibrator circuit is that the circuit can be in one of several different states (usually just two, which we ll call on and off for now). The prefix a- means not, so astable means the circuit is not stable in either of its states on or off and just keeps flipping back and forth between the two. We will do this with two transistors, which will constantly turn each other on and off, making the output wave form flip back and forth along with them. See Figure 6. With power applied to the circuit, both transistors will try to turn on due to the power flowing in via R 2 and R 3 to their bases. However, because nothing is perfect, including the manufacturing and materials that goes into making transistors, one of them will manage to do it before the other. (We re 11 [1.3.1; 07-OCT-2009]

12 Electronics Merit Badge Handout #2 1.2 Oscillators R 1 1K R 2 10K R 3 10K R 4 1K + B 1 9V Q 1 C µF C µF Q 2 Figure 6: Astable Multivibrator (Oscillator) Circuit Schematic talking about a tiny fraction of a second, but it s enough.) Let s say for the sake of our example here that Q 1 turns on first. This will open a path for power to flow to ground through Q 1 (collector to emitter), making Q 2 s base voltage low enough to turn it off. However, this power also starts charging up capacitor C 1. The time it takes to do this depends on the values of R 2 and C 1. As it charges up, the voltage going to the base of Q 2 rises until Q 2 turns on, allowing power to flow from its collector to ground and discharging C 1. Since Q 1 s base is also connected to that, this forces Q 1 to turn off. This starts charging up C 2, which will eventually turn on Q 1 and start the whole process over...and over... and over.... If we look at the collector of one of the transistors (let s say Q 2 ), we will see a square wave which goes to 0 V when Q 2 is on, and near +9 V when it s off. If the speed of the oscillator were very slow (perhaps 1 5 Hz), we could place an LED there and see it flash on and off. If the speed was within the audible range of the human ear, we could insert a speaker there and hear the square wave as a tone, as shown in Figure Using the 555 Chip Last time we introduced the 555 as an example of an integrated circuit (IC) chip. Its main function is to be a general-purpose timer or oscillator for other circuits. We can easily configure it to be a free-running oscillator by connecting it to an external resistor and capacitor whose values are chosen to provide the desired frequency of square wave on the output pin. (Full details on how to calculate these values were given in handout #1.) Figure 8 shows an equivalent oscillator to the circuit above, but this one uses only 5 parts instead of 8 (not counting the speaker or LED or whatever is attached to the output). In addition, the internal circuitry of the 555 is 12 [1.3.1; 07-OCT-2009]

13 1 AUDIO CIRCUITS Electronics Merit Badge Handout #2 R 5 R 1 1K R 2 10K R 3 10K 100Ω + B 1 9V R 4 47Ω Q 1 C µF C µF Q 2 Figure 7: Astable Multivibrator (Oscillator) Circuit with Speaker R 1 +9V 4 +9V 8 R 2 C Th Reset Vcc Discg Trig 555 Gnd 1 Ctrl Out 3 5 U 1 R 2 470Ω C µF D 1 Figure 8: 555 in Astable Oscillator Mode 13 [1.3.1; 07-OCT-2009]

14 Electronics Merit Badge Handout #2 1.2 Oscillators far more sophisticated than the simple circuit shown above, so it will make a better, more stable and reliable oscillator when used in an actual circuit. Note that in this version, we show an LED attached to the output (pin 3) instead of a speaker One-shot (Monostable) Multivibrator The free-running oscillator is the one we re most interested in for producing an audio signal, but as long as we re on the subject there are two variations of this circuit which are very useful to know about as well. We will see these used in digital and control circuits quite a lot. If an astable circuit constantly flips between on and off, and is therefore not stable in either state, then a monostable circuit is one which is stable in one of the two states. For example, if it s stable in the off state, it will hold a steady output at that state until something forces it to flip to the other state. It will stay there briefly and then flip back automatically to its stable state. We can control how long it will stay in the unstable state before flipping back over. This can be used to ensure a quick input pulse is lengthened to a standard duration. For example, suppose you have an electronic doorbell which needs to be triggered with a 1 second pulse while it makes its sound. If someone doesn t hold the button down that long, it won t work, and if someone holds it down continuously, it just gets annoying because it won t stop playing on its own. If the doorbell button activates a one-shot circuit, it will cause it to flip on. The one-shot will stay on (regardless of the button being pushed at this point) for a set length of time (say, 1 second) and then automatically flip off where it will stay until the button has been fully released and then pushed again. The circuitry is very similar to the free-running kind we saw above, but we remove capacitor C 1 (see Figure 9). We have also added an LED (D 1 ) as the output. The idea here is to have D 1 light up for a short period whenever button S 1 is pressed. Since C 1 is gone, the circuit will power up in a stable state with Q 1 off and Q 2 on. (The base of Q 1 is being grounded by Q 2, and nothing is trying to change that state at this point.) Since Q 1 is off, no power will flow through the LED, so it is also off. When the button S 1 is pressed, the base of Q 1 receives power to turn it on, allowing power to flow through the LED, turning it on. This also grounds the base of Q 2, turning it off. At this time, capacitor C 2 starts to 14 [1.3.1; 07-OCT-2009]

15 1 AUDIO CIRCUITS Electronics Merit Badge Handout #2 D 1 S 1 R 3 R 4 R 1 R 2 + B 1 9V Q 1 R 5 C 2 Q 2 Figure 9: Monostable Multivibrator (One-Shot) Circuit Schematic charge. When it does, Q 2 will turn on again, shutting Q 1 off and bringing the circuit back to a stable state where it will remain until S 1 is pressed again. Incidentally, we can configure a 555 to be a one-shot circuit as well, by simply rearranging how the connections to the chip are made (see Figure 10). +9V +9V R 1 +9V 10K R 3 S C 1 Th Reset Vcc Discg Trig Gnd 1 8 Ctrl Out 3 5 U 1 R 2 470Ω C µF D 1 Figure 10: 555 in Monostable Multivibrator Mode The biggest change is attaching pushbutton S 1 to pin 2, which is the 555 circuit s trigger input to start the next cycle. In the astable mode (the circuit in the previous section), we hard-wired it so the 555 would keep re-triggering itself forever. 15 [1.3.1; 07-OCT-2009]

16 Electronics Merit Badge Handout #2 1.2 Oscillators Flip-flop (Bistable Multivibrator) Carrying on with the variations in multivibrator circuits, we come to the bistable variety, which means it is stable in either of its states. This means we can set it to on and it will keep that state continuously, outputting an on signal level, until we turn it off which will make it output an off signal level until we tell it to go back on again. These circuits, commonly called flip flops, are one of the fundamental logic components used to make digital circuits which store and manipulate information. A single flip flop is able to store one bit of information. By combining them together, we can store more bits of information. A computer s RAM memory is really just a massive array of flip flop circuits. To modify our circuit to be a flip flop (as we do in Figure 11), we need D 1 R 1 Q S 1 Reset R 2 Q R 5 Q 1 S 2 D 2 Q Set R + 3 R 4 Q R 6 Q 2 B 1 9V Figure 11: Bistable Multivibrator (Flip-Flop) Circuit Schematic to remove the other capacitor (C 2 ), so there is no circuitry to automatically change state after a time delay. We will include two LEDs this time, so you can see when the circuit is in each state. We will call one state Q and the other will be Q. 2 We could attach connections to other circuits at the arrows labelled Q and Q if we wanted to use those signals to control something else. When this circuit is powered on, one of the transistors Q 1 or Q 2 will come on first, forcing the other off (as we ve seen with the previous multivibrator circuits). Let s say Q 1 turns on first. This will open a pathway for current to flow from output Q to ground (in other words, signal output Q is off ). This also means current is flowing through LED D 1 (labelled Q ), and that 2 This is a common notation for logic circuits. Putting a bar over a signal name indicates that it has the opposite meaning than it normally would. So if Q is the output which is on (at high voltage) when the circuit is in the on state, then Q (which is pronounced not Q ) is on when Q is off, and vice versa. 16 [1.3.1; 07-OCT-2009]

17 1 AUDIO CIRCUITS Electronics Merit Badge Handout #2 the base of Q 2 is grounded, turning that transistor off as well. A positive voltage is therefore seen at output Q, from the positive power supply via R 4, so we say signal output Q is on. This positive voltage is also sent to the base of Q 1, keeping it locked on. We say the flip-flop is reset. When we press the Set button (S 2 ), we cause a positive voltage to appear at the base of Q 2, turning it on. This in turn grounds the base of Q 1, turning it off and extinguishing LED D 1. Now the positive voltage being supplied via R 1 to the base of Q 2 keeps it turned on (which keeps Q 1 off). This also turns on LED D 2 (labelled Q ), and putting a positive voltage at output signal Q. Output signal Q is now at 0 V. The flip-flop is now set. Pressing the Reset button (S 1 ) flips the circuit to the reset state as described above. There are many varieties of flip flop IC chips which can be used in your own circuits. Since they are really much more employed in digital circuits than audio, we will discuss flip-flops further in the digital circuit section below. 1.3 Amplifiers Whether we produce our own audio signal with an oscillator, or pick up an external sound using a microphone, we may need to amplify that signal to make it loud enough to be heard. It is often the case that the signal levels used internally by audio circuits are not strong enough to actually move a speaker, so some sort of amplifier is required to boost the signal at the final output Transistors In the simplest case, we can use a transistor as a kind of amplifier. Transistors are not always fully on or off, but can be adjusted between those two extremes. The more voltage at the transistor s base, the more that will be allowed to flow between the collector and emitter. To use an analogy, this is like having a remote-controlled valve in a water pumping system. A small turn of a dial on the control panel may open a huge flood gate and let a large quantity of water flow. With a transistor, a very small fluctuation in base voltage controls the valve gate which allows a much larger voltage change on the output. Since the output change is proportional to the input change, the end result is that the signal is amplified it is the same shape and frequency but has a greater 17 [1.3.1; 07-OCT-2009]

18 Electronics Merit Badge Handout #2 1.4 Example Circuit: Morse Code Practice Oscillator amplitude (it is a stronger, higher voltage signal). Figure 12 illustrates the general idea. Figure 12: Transistor Amplifying an Audio Signal Operational Amplifiers (Op-Amps) There are a number of good amplifier ICs available. Some are designed to be high-gain audio amplifiers, some are designed for the radio-frequency signal range. Others are used to amplify lower-frequency signals or even to compare voltages. One basic general-purpose amplifier chip is the 741 operational amplifier (or op amp ). Like the 555 timer, it has a thousand and one uses, depending on how it is configured and used in larger circuits. 1.4 Example Circuit: Morse Code Practice Oscillator This circuit would be acceptable as a project for requirement #4(c). This circuit (Figure 13) uses a 555 chip arranged as an oscillator to produce a steady Hz tone. We use a pushbutton to complete the circuit, so it only makes a sound while the button is pressed. If we use a telegraph key, we can use this circuit to practice sending Morse code. If we add a variable resistor next to R 2, we can adjust the frequency of the tone (since that is controlled by the combination of values of the resistor/capacitor pair). Here we have a 100 kω variable resistor R 3 in series with the 47 kω fixed resistor R 2, so if R 3 is dialed all the way down to 0 Ω, the resistance controlling the frequency of the 555 will be 47 k. If it is dialed all the way up to 100 k, then the resistance will be 147 k. With the values for R 1 and C 1 as shown, this gives the circuit a range from Hz (between B and B in the fourth octave) up to Hz (just flat of F in the sixth octave). The modified circuit is shown in Figure [1.3.1; 07-OCT-2009]

19 1 AUDIO CIRCUITS Electronics Merit Badge Handout #2 R 1 10K 4 8 Reset Vcc S 1 Power + 9V B 1 R 2 C 1 47K 0.01µF Discg Th 555 Ctrl 5 Out 3 Trig Gnd 1 + C 2 33µF SP 1 S 2 Key Figure 13: Example Circuit: Morse Code Practice Oscillator R 1 10K 4 8 Reset Vcc S 1 Power + 9V B 1 R 2 R 3 47K Discg Th 555 Ctrl 5 Out 3 Trig Gnd + C 2 33µF SP 1 C 1 100K 0.01µF 1 S 2 Key Figure 14: Example Circuit: Morse Code Oscillator with Tuning Control 19 [1.3.1; 07-OCT-2009]

20 Electronics Merit Badge Handout #2 1.4 Example Circuit: Morse Code Practice Oscillator If we want to add a volume control, we just put a variable resistor in line with the speaker to reduce the output voltage as the knob is turned. See Figure 15. R 1 10K 4 8 Reset Vcc S 1 Power + 9V B 1 R 2 R 3 47K 100K Discg Th 555 Ctrl 5 Out 3 Trig Gnd 1 + C 2 33µF R 4 10K Vol. SP 1 C µF S 2 Key Figure 15: Example Circuit: Morse Code Oscillator with Volume Control 20 [1.3.1; 07-OCT-2009]

21 2 CONTROL CIRCUITS Electronics Merit Badge Handout #2 2 Control Circuits To pass requirement #4(a), you need to describe how electronics can be used for a control purpose, but don t need to be able to go into the detail you ll see below. If you wish to build a control circuit for your requirement #4 project, you may wish to read this section fully so you can be able to describe how your project works. Electronic circuits are quite often used to control devices (for example, a sensor may detect you have walked up to an automatic door at your grocery store, and activate the door s opening device). Some of them even control us (a traffic light tells drivers when to stop or go). Control circuits usually have some sort of input telling them when to start controlling whatever they are connected to. This could be a sensor which detects something happening in the real world (for example, a temperature sensor which activates a thermostat to turn on your heater or air conditioner). Or it might be a switch activated manually by a person (such as when you press an elevator button to tell the elevator s control circuit to move the elevator to another floor). A control circuit could even just contain its own internal timer telling it when to do something, without input from outside (for example, a traffic light controller although many of those also use sensors in addition to their timers). 2.1 Sensors A sensor is a device for signalling some event in the environment to which the circuit needs to respond. A few common varieties will be described here Manual Input If the circuit can be controlled by a human telling it what to do, this is usually arranged by making a switch or pushbutton available to be pressed. This is connected to a voltage source so that when the switch is closed, it allows current to flow into a circuit. Look at the description of the flip flop circuit above for an example of how a circuit changes state in response to a human pushing buttons. More sophisticated input sensors could include a numeric keypad such as you would have on a telephone or calculator, or even a full keyboard to allow you to type text messages into the device. 21 [1.3.1; 07-OCT-2009]

22 Electronics Merit Badge Handout #2 2.1 Sensors Impact Sensors A special kind of switch is designed to be closed when the device comes in physical contact with something. This could be to sense if something bumps into it, or (in the case of a robot, for instance) if the device itself bumps into a wall or other object. Electrically, though, this is identical to a pushbutton when activated, it closes a pair of contacts allowing current to flow into part of the circuit Light Sensors Some electronic components are sensitive to light falling on them. There are special kinds of resistors, diodes, and transistors, for example, which change how they work depending upon whether light is shining on them or not. In the schematic symbols for these parts, there are two arrows pointing in to the part to indicate that they are affected by radiation (in this case, light) hitting the part. 3 In the case of a light-dependent resistor (or LDR for short), the amount the resistor impedes the flow of current through it depends on how much light hits it. The more light, the less resistance (so more current will flow). In this way, a control device can be sensitive not only to the presence or absence of light, but even to the amount of light. A circuit might use this to turn on your porch light when the sunlight drops below a certain level, and then off again in the morning when it s light enough outside. A photodiode, by comparison, will conduct current in one direction (from anode to cathode, or in other words, in the direction of the arrow 4 ) just like a normal diode, but only when light hits it. When it s dark, it will not conduct current at all, in either direction. Some photodiodes are sensitive to visible light, while others only respond to infrared light. This allows them to detect an infrared beam (perhaps responding when that beam is interrupted by a passing object) without being confused by the ambient room lighting. There are also phototransistors. Typically, these have only a collector and emitter lead. Instead of using a base lead to control the transistor using 3 Compare this to the symbol we have already seen for LEDs, which has two arrows pointing away from the device, indicating that radiation (light, again) is emitted by the device. 4 Assuming positive-to-negative current flow. 22 [1.3.1; 07-OCT-2009]

23 2 CONTROL CIRCUITS Electronics Merit Badge Handout #2 voltage, the phototransistor turns on or off in response to how much light is falling on it Motion and Proximity Sensors Another common kind of sensor detects when a person or other moving object moves near it. Some of these use heat to sense a living creature moving nearby. Others send out ultrasonic pulses and listen for echoes to track movement. Still others create a weak radio field which will be disrupted by a person walking through it Temperature Sensors There are a variety of temperature-sensing devices. Some are manufactured as chips in packages which look identical to a transistor. Others are made from a pair of metals which will change their resistance as the metals expand or contract due to temperature changes. In most cases, the sensor looks like a resistor to the circuit, which changes how much current will flow through it depending on the temperature around the sensor. 2.2 Relays Once a control circuit has discovered something (perhaps in response to input from a sensor), it may need to turn on or off some external device. For example, a daylight sensor may turn on or off your porch light. Usually the device being turned on or off uses much more power than the sensor can control directly, so some sort of relay is needed. A relay is like a remote-controlled switch. A small current is used to turn the switch on or off, while the switch itself is capable of controlling a much more powerful signal. We have already seen that transistors can be used like this if the amount of power being controlled isn t too large. 5 There is also a component called a relay which is a physical switch (with mechanical contacts that open and close). Instead of a person manually flipping the switch contacts, though, they are moved by an electromagnet. When a current passes through the coil (shown at the bottom of the schematic symbol here), it pulls the contacts down, making contact. When that current stops, a spring pulls the contacts back apart again. Just like other switches, relays may come in SPST, SPDT, DPST, DPDT, and other 5 Although there are larger, high-power transistors designed for that purpose, too. 23 [1.3.1; 07-OCT-2009]

24 Electronics Merit Badge Handout #2 2.2 Relays Figure 16: Schematic Symbol and Photo of Photodiode Figure 17: Schematic Symbol and Photo of Phototransistor Figure 18: Schematic Symbol and Photo of Relay 24 [1.3.1; 07-OCT-2009]

25 2 CONTROL CIRCUITS Electronics Merit Badge Handout #2 numbers and combinations of contacts. SPDT and DPDT relays are both very common. 2.3 Example Circuit: Light-Sensitive Relay This circuit would be acceptable as a project for requirement #4(a). This circuit will activate a relay when light hits its sensor, and release the relay when it is dark. Since a mechanical relay is used, it can control any kind of load as long as the limitations of the relay and wiring are not exceeded in terms of volts, amps, etc. (See Figure 19.) K 1 6 9V DC Relay S 1 + 9V B 1 Q 1 R 1 D 1 1N4002 Q 2 2N K Figure 19: Example Circuit: Light-Sensitive Relay 25 [1.3.1; 07-OCT-2009]

26 Electronics Merit Badge Handout #2 3 Digital Circuits To pass requirement #4(b), you need to describe, in your own words and in simple terms, the basic digital electronics principles described in this section. You should be able to describe the representation of true and false values as 1s and 0s or as positive voltage and ground on wires. You should be able to talk about logic gates such as AND and OR gates, and describe what a truth table is. You don t need to be able to go into the level of detail you ll see below. If you wish to build a digital circuit for your requirement #4 project, you may wish to read this section fully so you can be able to describe how your project works. Digital circuits deal, for the most part, with processing information by encoding, storing, moving, transforming, and reporting data. The data are encoded (usually) as patterns of 0s and 1s. Within the circuit, these two states may be encoded as (for example) +5 V to represent 1 and 0 V to represent 0. On magnetic tapes or discs, magnetic bits recorded one direction or another represent 1s and 0s. On compact discs, the presence or absence of tiny dots represent these bits of information. A single bit 6 is enough to store a single fact, such as the door is currently open. Supposing a digital circuit is monitoring a door sensor, we might use a bit which is set to 1 to indicate that this fact is true (the door is currently open), or 0 to mean that it is false (the door is not currently open). Electronically, this might be represented as +5 V to indicate a 1 or true value, and 0 V for 0 or false. We can also encode more complex data by using multiple bits at once. A simple numeric value could be directly encoded by using one bit (an electrical signal on a single wire) for each digit of the number, if that number is represented in binary (base 2). The first eight are shown in Table 1. We can actually encode anything as a pattern of bits. A personal computer s circuits, for example, might use an encoding scheme known as ASCII 7 to determine what bit pattern is used to represent the symbols which appear on your screen, an example of which is shown in Table 2. 6 The term bit is a contraction of binary digit 7 The American Standard Code for Information Interchange 26 [1.3.1; 07-OCT-2009]

27 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 Number Bit Pattern Table 1: Binary Bit Patterns for 0 7 Symbol Bit Pattern Symbol Bit Pattern A ! B C * D E F Table 2: Binary Encoding of ASCII Symbols 27 [1.3.1; 07-OCT-2009]

28 Electronics Merit Badge Handout #2 3.1 Binary Numbers 3.1 Binary Numbers To pass requirement #4(b), you need to be able to convert numbers between binary and decimal (in both directions, with numbers your counselor will give you). The explanations about working in octal (base 8) and hexadecimal (base 16) are provided here to help explain the concept better, and are often used in digital electronics work. However, you do not need to know or demonstrate anything with octal or hexadecimal numbers for the merit badge. The paterns of 1s and 0s manipulated by digital circuits generally represent numbers (as discrete symbols or numeric quantities). Since we only have two digits available (0 and 1), these numbers are expressed in base 2, also known as binary. Humans probably due to having ten fingers to count with have used base 10 (or decimal ) systems to represent numbers. If we think of how the decimal system works, we can see how the familiar concepts there apply to binary numbers and can even do arithmetic in binary Number Base Basics Since we re thoroughly familiar with how decimal (base 10) numbers work, this will seem almost too obvious to detail here, but while reading through this description, look past the familiar and see how the mechanics of working with digits representing a number work in general and particularly how all this might work if using a number base other than 10. In any number base, each digit position represents a power of that base. The digit there is how many of the corresponding power to add to arrive at the number being represented. The right-most digit is the units, ones, or 0-power position. With base 10, then, this represnts multiples of Here we count through each of the digits from ,1,2,3,4,5,6,7,8,9 To advance one more number, we have to start the next digit position to the left, which represents the next higher power of the base. In this case, 10 1, or the tens position. Putting a 1 in this new position, we cycle back to 0 in the units position, and can keep counting through the units digits 0 9 just like before, although this time 10 is added to each value: 10,11,12,13,14, 15,16,17, 18,19 28 [1.3.1; 07-OCT-2009]

29 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 And we can continue by incrementing the tens digit every time we have to cycle back the units digit from 9 back to 0: 19,20,21,...,29,30,31,...,98,99 Every time all the digits are the highest digit in our system (9), we just add a 1 in a new position, representing the next higher power of our base (10), which in this case is 10 2, or the hundreds place, and start the pattern over again: 100,101,102,...,109,110,111,...,998,999 In this way, we can continue indefinitely, adding digits for 10 3, 10 4, and so on. To understand the meaning of a set of digits such as 512, we simply need to know that each digit represents a multiple of a power of our base. In other words, 512 means: 512 = ( ) + ( ) + ( ) = (5 100) + (1 10) + (2 1) = = Same Tune, Different Verse (Octal) We can apply those same concepts to any base. Suppose we use base 8 (called octal ), instead of base 10. This works just as before, but we only have eight digits (0 7) rather than ten. So when we count, we can only go eight steps before we need to advance to the next digit. 0,1,2,3,4,5,6,7 This works as we ve already seen, but each digit represents a multiple of a power of eight, so when we add the next digit, it represents 8 1, or eights. 10,11,12,13,14,15,16, 17,20,21,... It may look strange at first to jump from 17 to 20, but we re just following the same pattern we did with base 10, but now we only have eight digits to work with. Reaching the highest value we can express with two digits (77), we add another digit, this time representing the 8 2 place, and keep going: 77,100,101,...,107,110,111,...,176,177,200, [1.3.1; 07-OCT-2009]

30 Electronics Merit Badge Handout #2 3.1 Binary Numbers If it s not obvious from context which number base we re using, we put the base as a subscript at the end of the number. So means the number 52 in base 10, while 52 8 means the number 52 in base 8. As before, to understand the meaning of a set of digits such as , we simply need to know that each digit represents a multiple of a power of 8. So, in base 8, the digit sequence 4723 means: = (4 8 3 ) + (7 8 2 ) + (2 8 1 ) + (3 8 0 ) = (4 512) + (7 64) + (2 8) + (3 1) = = So we see that = This also illustrates how to convert a number from octal to decimal. Multiply out each digit by the corresponding power of 8, and add their values together Base 16 (Hexadecimal) Since digital circuits are based on the binary number system, it is often convenient to work in base 8 and base 16 (as those bases are themselves powers of 2). Base 16, or hexadecimal, is the more commonly used of the two. Since we need 16 digits for this base, we add the letters A F to the standard digits 0 9: 0,1,2,3,4,5,6,7, 8, 9,A,B,C,D,E,F See Table 13 for another example of how hexadecimal numbers look, and how the letters A F stand in for the digits Each digit represents a power of 16, but otherwise the system works just like the others: F,10,11,...,1F,20,21,...,FE,FF,100,101,... Converting a hexadecimal number to decimal works just the same as it does for octal, except for the change in base. So to convert 2B6 16 to decimal, we multiply out the powers of 16, and remembering that the digits A F represent the values 10 15: 2B6 16 = ( ) + ( ) + ( ) = (2 256) + (11 16) + (6 1) = = So 2B6 16 = [1.3.1; 07-OCT-2009]

31 3 DIGITAL CIRCUITS Electronics Merit Badge Handout # Binary (Base 2) Having seen examples with bases 10, 8, and 16, it should be easy to see how base 2 is just the same idea, only with powers of 2, and a digit set consisting of only two digits: 0 and 1. When we count, we run out of digits faster (having only two of them), so the number of digits builds rather quickly: 0,1,10,11,100,101, 110, 111,1000, 1001, 1010,1011,1100,... Converting a binary number to decimal is done exactly the same as with octal or hexadecimal numbers, only we use powers of 2 instead of 8 or 16. For example, the binary number is converted like this: = (1 2 3 ) + (0 2 2 ) + (1 2 1 ) + (1 2 0 ) = (1 8) + (0 4) + (1 2) + (1 1) = = Therefore, = Converting From Base 10 Converting numbers the other direction is just the reverse of what we ve seen. Starting with a number in base 10, but instead of multiplying each digit out, we ll divide them Decimal to Octal Let s see what the decimal number would be in octal (base 8). In octal, each digit represents a power of 8, so we will repeatedly divide the number by 8 and save off the remainders. To begin with, = 15, with a remainder of 3. We save off the remainder, which will be the units digit of our answer: = 15 remainder 3 Now we take the result of the previous division (15) and try to divide it by 8 again = 1, with a remainder of 7, so we save off the remainder and divide the left over amount again. 1 8 = 0, with a remainder of 1. Since we now are down to zero, we have nothing left to divide and we can 31 [1.3.1; 07-OCT-2009]

32 Electronics Merit Badge Handout #2 3.2 Converting From Base 10 stop = 15 remainder = 1 rem = 0 rem We collect the remainders as the digits of our answer: = We can check our work by reversing it again: = (1 8 2 ) + (7 8 1 ) + (3 8 0 ) = (1 64) + (7 8) + (3 1) = = Decimal to Hexadecimal Converting decimal to hex (base 16) is the same, only we divide by 16s along the way. Let s convert the decimal number to hex: = 24 remainder = 1 rem = 0 rem So the remainders are 1, 8, and 13. Since the hexadecimal system uses 16 digits, we use A F to represent the digit values (see Table 13 on page 57), so the digit for 13 is D. Therefore, = 18D 16 On your own, figure out what the number is in hexadecimal. If you re ready with the answer during the next session, you ll get a bonus prize. 32 [1.3.1; 07-OCT-2009]

33 3 DIGITAL CIRCUITS Electronics Merit Badge Handout # Decimal to Binary Converting between decimal and binary (both directions) is the only required conversion you need to demonstrate for the merit badge. After practicing with octal and hexadecimal, though, this should be a snap. Let s try converting the decimal value 6 10 to binary (base 2): 6 2 = 3 remainder = 1 rem = 0 rem So now we know that 6 10 = Binary Logic You should be familiar with the general topic of logic gates such as AND gates, OR gates, etc. but don t need to be able to go into great detail about their operation, for the purposes of passing requirement #4(b). We use binary values to represent the two logical states true and false. A single fact is true or false at any given time. As we will see, these simple true/false values can be combined using logical operators (in a similar way to how we combine English sentence fragments with conjunctions like and, or, and but ) to make more complex decisions. A simple value which is either true or false is also called a boolean value in electronics and computer science, and the ways we can mathematically manipulate boolean expressions is called Boolean Algebra. I will give a bonus prize next time to a scout who can tell me where we get the name Boolean from. A special set of electronic components called logic gates perform basic decision-making operations and math calculations by performing simple logical functions on collections of bits. We will look at the basic logic gates one at a time. Each is basically a conjunction for combining bits, and in fact they are named after the English conjunctions we use to combine sentences. 33 [1.3.1; 07-OCT-2009]

34 Electronics Merit Badge Handout #2 3.3 Binary Logic YES Gates (Buffers) Buffers don t provide much in the way of logic decisionmaking, but they help interface different circuits together, boost the power output from other logic gates, and isolate signals. A Out The truth table is shown in Table 3. The 4050 chip is one example, Input A Output (A) Table 3: Truth Table for A (Identity Function or YES Gates ) which has six buffers as shown in Figure Vcc Gnd Figure 20: Chip Pinout Diagram: 4050 Hex Buffer NOT Gates (Inverters) A NOT gate takes a single input and outputs its inverse (or opposite). This is similar to the English usage of the word not. For example, the sentence, It is not raining A Out 34 [1.3.1; 07-OCT-2009]

35 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 now is the opposite of the sentence, It is raining now, in that whichever one happens to be true, the other will be false. The truth table for the NOT gate, which is also called an inverter, is very simple (see Table 4). Input A Output (A) Table 4: Truth Table for A (NOT A) The 4049 chip has six inverters as shown in Figure 21: Vcc Gnd Figure 21: Chip Pinout Diagram: 4049 Hex Inverter Notice the little circle on the gate s output symbol. This circle indicates that the normal sense of the signal is inverted. Without that circle, you d have a gate that outputs whatever is presented on its input, which is called [1.3.1; 07-OCT-2009]

36 Electronics Merit Badge Handout #2 3.3 Binary Logic OR Gates One of the simplest binary logic operations is OR, which says that the result of the whole sentence is true if any one of the input facts is true. In the English sentence, If it s before 6:00, or Saturday, then go back to sleep, a decision ( if...then ) is being made based on whether fact A (it s before 6:00) or fact B (it is Saturday) is true. If either of them are true, we ll stay in bed. We can represent the possible combinations of facts and the resulting decision on a table like Table 5, below. Before 6:00? Saturday? No No No No Yes Yes Yes No Yes Yes Yes Yes A B Go back to sleep! Table 5: Decision Table for Going Back to Sleep Out Electronically, we could have two wires, A and B, which represent the current state of each of the facts in our sentence. Connecting them to the inputs of an OR gate, the gate s output will be true if either A or B is true (or both), but false if neither A nor B are true. We can show this in a general table of inputs and outputs called a truth table, using 0 to represent false and 1 to represent true. The truth table for A or B (also written as A + B ) is shown in Table 6. Inputs Output A B (A+B) Table 6: Truth Table for A + B (A OR B) Here s an example circuit showing an OR gate in action. (Figure 22.) A 36 [1.3.1; 07-OCT-2009]

37 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 +5V S 1 S 2 U D 1 R 3 220Ω R 1 10k R 2 10k Figure 22: Example of OR Gate Circuit common OR gate component is the 4071, which is a 14-pin chip containing 4 2-input OR gates in one package. When buttons S 1 or S 2 are pressed, the corresponding input line is connected directly to the +5 V power supply, which means that input is true. If either of these are true, the OR gate will produce a +5 V ( true ) output, which flows through the LED to ground, lighting it up. If neither input is true (neither button is pressed), then the OR gate s output will be at 0 V ( false ) so there is no power flowing through the LED. One thing of note with this circuit is the pair of resistors R 1 and R 2. These are called pull-down resistors and are necessary because a logic input must always be definately either a 0 or 1 (on or off, +5 V or 0 V) at any given time. If we press the button, the line it is connected to is definitely a 1 (+5 V), but if we let go of the button, it wouldn t be connected to anything, so its state would be random and unpredictable. By connecting the inputs to ground, they are definitely at 0 V by default. We can t just connect them to ground directly, or we would have a short circuit when the buttons got pressed! This way, the direct connection to +5 V through the button is the path of least resistance the current will flow along, but without it, there s still enough of a connection to ground to pull the input line down to ground. The 4071 chip itself has 14 pins and 4 OR gates, arranged like shown in Figure 23. The power supply for the chip itself is on pins 7 and 14, and should be in the range 3 15 V. (The chips we will show here are CMOS chips which allow greater flexibility in the range of operating voltages in fact, we will power our examples from 9 V batteries, although 2 3 AAAs would also 37 [1.3.1; 07-OCT-2009]

38 Electronics Merit Badge Handout #2 3.3 Binary Logic Vcc Gnd Figure 23: Chip Pinout Diagram: 4071 Quad 2-input OR Gate work. Another common kind of digital chip is called TTL. These require less careful handling but need to be powered near 5 V so they re less flexible. For comparison, the TTL equivalent to the 4071 is the 7432 but although they are both quad 2-input OR gate chips, the arrangement of pins are quite different; you cannot just directly substitute one type for the other.) Since we only need a single OR gate for our circuit, we ll choose the one with inputs on pins 12 and 13, and output on pin 11. It is important not to leave any gate inputs floating free. You should connect them to power or ground. We ll ground them here. The circuit would be wired up like shown in Figure AND Gates An AND gate works just like the word and in English. It takes a set of two or more bits as its inputs, and produces a single output bit which is true if, and only if, all of the inputs are true. A BC Out We can represent all the possible combinations of inputs and outputs with a truth table like Table 7 (assuming a three-input AND gate, with inputs labelled A, B, and C ). The 4081 chip has four 2-input AND gates in one package (see Figure 25). There are many other kinds of AND gate chips available, including the 4073 which has three 3-input AND gates (see Figure 26). 38 [1.3.1; 07-OCT-2009]

39 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 +5V Vcc Gnd Figure 24: Example of OR Gate Circuit with Chip Connections Inputs A B C Output (ABC) Table 7: Truth Table for ABC (A AND B AND C) 39 [1.3.1; 07-OCT-2009]

40 Electronics Merit Badge Handout #2 3.3 Binary Logic Vcc Gnd Figure 25: Chip Pinout Diagram: 4081 Quad 2-Input AND Gate Vcc Gnd Figure 26: Chip Pinout Diagram: 4073 Triple 3-Input AND Gate NOR and NAND Gates It is common that the output from an OR or AND gate needs to then be inverted. Instead of monitoring a signal which means, if the door is open and the light is on, you A BC Out might want to have the circuit see the opposite: if it is not true that the door is open and the light is on (or, to rephrase that, if the door is closed or the light is off ). To help accomplish this, there are gates which are a A BC Out combination of one of the other logic types plus an inverter. These are Not-AND or NAND and Not-OR or NOR gates. Table 8 summarizes the outputs from both types of gates. The 4001 chip is a standard NOR gate (Figure 27), and the 4011 chip is a standard NAND gate (Figure 28). 40 [1.3.1; 07-OCT-2009]

41 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 Inputs NOR Output NAND Output A B C (A+B+C) (ABC) Table 8: Truth Table for NOR and NAND Vcc Gnd Figure 27: Chip Pinout Diagram: 4001 Quad 2-Input NOR Gate Vcc Gnd Figure 28: Chip Pinout Diagram: 4011 Quad 2-Input NAND Gate 41 [1.3.1; 07-OCT-2009]

42 Electronics Merit Badge Handout #2 3.3 Binary Logic XOR (Exclusive-OR) Gates The OR gate we described above is more formally called an inclusive OR gate, because it will output a true value (1) if any input is true, including the case where more A BC Out than one is true. By contrast, there is also an exclusive OR (called XOR for short) which means one and only one input may be true. Its schematic symbol is shown here on the top, A BC Out and the negated version ( Exclusive-NOR or simply XNOR ) on the bottom. Table 9 summarizes the outputs from both of these gates: The 4070 chip Inputs XOR Output XNOR Output A B C (A B C) (A B C) Table 9: Truth Table for A B C (A XOR B XOR C) and Its Inverse is a standard XOR gate (see Figure 29). And the 4077 chip is a standard Vcc Gnd Figure 29: Chip Pinout Diagram: 4070 Quad 2-Input XOR Gate XNOR gate (see Figure 30). 42 [1.3.1; 07-OCT-2009]

43 3 DIGITAL CIRCUITS Electronics Merit Badge Handout # Vcc Gnd Figure 30: Chip Pinout Diagram: 4077 Quad 2-Input XNOR Gate 3.4 Binary Manipulation Circuits You do not need to know anything in this section to meet the merit badge requirements, unless needed to explain the operation of your project. You may find this information interesting or useful, however. In addition to simple logic gates, there are more complex digital components which can do all sorts of things with binary information. A few basic ones will be described here: Counters A counter circuit, such as the 4510 pictured in Figure 31, counts pulses on U/D 15 Clk Vcc 3 D C B 4 A 4510 D 2 C 14 B 11 A 6 1 Preset 9 Reset 5 CI Gnd CO 7 Figure 31: Schematic Symbol for 4510 Decade Counter Chip its clock input pin. When the chip is reset, its four binary outputs are all reading 0. Every cycle of the clock pin from 0 to 1 causes the chip to count up by one, outputting the next binary number in the sequence on the 8 43 [1.3.1; 07-OCT-2009]

44 Electronics Merit Badge Handout #2 3.4 Binary Manipulation Circuits output pins. The 4510 is a decade counter or in other words, it counts from 0 9. After reaching 9, the next clock cycle takes it back to 0 and it keeps counting up again from there. The output sequence would look like Table 10. Input Outputs Decimal Clock Carry D C B A Value (initial state) (next cycle) (next cycle) (next cycle) (next cycle) (next cycle) (next cycle) (next cycle) (next cycle) (next cycle) (next cycle) (next cycle) Table 10: Truth Table for 4510 Decade Counter Chip The counter has some particular features which give quite a bit of flexibility in how it can be used. Pin 10 controls the direction it will count. If this is set high ( true ), it will count up from 0 9, but if set low ( false ) it will count down from 9 0. Pins 1, 3, 4, 12, and 13 allow the counter to be preset to a particular starting value instead of zero. If pin 1 is set to high, the binary value input on pins 3, 4, 12, and 13 becomes the current counter value and is output. The counting will proceed from there. For normal operation, pin 1 is set low. Pin 9 is normally kept low, but if it is raised to a high value it resets the counter to zeroes. Every time the counter reaches the end of its range, the carry output goes to 1 on pin 7. If this is fed to the clock input (pin 15) of another counter chip, the two counters work together (the second one will become the tens digit of a two-digit display. It s carry bit could be sent to the third counter, and so forth, for as many digits as are needed. This connection of one counter to another is called cascading. 44 [1.3.1; 07-OCT-2009]

45 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 There is another chip, the 4516, which is identical to the 4510 except that it counts all the way from 0 15 (fully using all four bits of output) Decoders Some chips encode a set of bits into binary, or decode them back again. For example, the 4028 chip shown in Figure?? is a BCD (binary-coded decimal) decoder. It takes a 4-bit binary value between 0000 and 1001, and turns on D C B A Vcc 4028 Gnd Q9 5 Q8 9 Q7 4 Q6 7 Q5 6 Q4 1 Q3 15 Q2 2 Q1 14 Q0 3 8 Figure 32: Schematic Symbol for the 4028 BCD-to-Decimal Decoder one of ten output lines as a result. This allows a digital system to select one of ten devices to activate. An example using this to sequence a set of ten LEDs is shown below. The truth table for this chip looks like Table Digital I/O (Input/Output) Switches The specifics of pull-down or pull-up resistors and mechanical switch debouncing circuits is important to know and might be necessary to explain your project circuit. Other than that, this is not specifically required for you to know in order to pass the merit badge requirements. Switches may be used to manually set input signals into a circuit. For example, we may use switches to input the initial counter value of the 4510 chip, or to control the reset or clock lines. However, we need to be careful of two things. First, we need to be sure that an input is always tied to positive voltage (providing a logic 1 ) or to ground (providing a 0 ). The usual way to 45 [1.3.1; 07-OCT-2009]

46 Electronics Merit Badge Handout #2 3.5 Digital I/O (Input/Output) Inputs D C B A Outputs Q9 Q8 Q7 Q6 Q5 Q4 Q3 Q2 Q1 Q Table 11: Truth Table for 4028 Decoder Chip arrange this is to use a pull-up or pull-down resistor to pull the line to one logic state while the switch or pushbutton connects it temporarily directly to the other signal state. For example, the pushbutton shown in Figure 33 will take the input line to ground ( low, 0, or false value) when pressed. When the button is released, the resistor will pull the line up to high voltage ( 1 or true value). +5V 10k Figure 33: Simple Pushbutton Circuit with Pull-up Resistor The other concern is switch bounce. Mechanical switches such as toggle switches, pushbuttons, and keyboard keys do not make a completely clean connection when pressed. There is a point, for the tiniest fraction of a second as the contacts come together or apart, when a small spark jumps between them. If you have sensitive enough equipment you could notice that it behaves as though the swtich were opened and closed several times very, very, very quickly. Unfortunately, these digital circuits can be that sensitive, so we usually add some extra components to debounce the 46 [1.3.1; 07-OCT-2009]

47 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 switches, smoothing out the transition between on and off to compensate for this effect. A circuit such as the one in Figure 34 is usually sufficient. This uses a +5V 10k 10µF Figure 34: Simple Debounced Pushbutton Circuit capacitor, along with a pair of inverters (for example, any two gates out of a chip such as the 4049 we discussed on page 35), to smooth out the bounce from the button. Since we have two inverters, the logic sense of the output is the same as if we just had the pushbutton alone (like in the previous diagram), but it takes a very small amount of time for the signal to travel through the inverters (a number of nanoseconds, usually), during which the inverter gates will be holding the output line at a steady level until they see a solid signal level change on their input side Discrete LEDs For output of single bits of information, you can simply hook up an LED such that a positive signal ( 1 ) will flow through the LED to ground (or, if you want it to light up on a 0 level signal, hook up the LED the other way around). However, it is often the case that a logic circuit won t have enough power to spare to light up an LED, so you should route the signal through an inverter or buffer which should be able to power the LED just fine Segment LEDs If a digital display is needed, a common output device is the 7-segment LED display (see photo in Figure 36). This is electrically identical to 7 LEDs connected together (see Figure 36). Either the LED anodes or the cathodes may be tied together. The displays will be marked as common anode or common cathode accordingly. 47 [1.3.1; 07-OCT-2009]

48 Electronics Merit Badge Handout #2 3.5 Digital I/O (Input/Output) Figure 35: Photo of 7-Segment LED Display (Single Digit) a a b c d e f g f g b common e d c Figure 36: 7-Segment LED The LED segments themselves are arranged in the pattern shown above. By lighting them in certain combinations, the digits from 0 9 may be displayed. It is common for the letters A F to be displayed for output of hexadecimal numbers. To make this work on a 7-segment display, a mixture of upper- and lower-case letters is used, usually A, b, c (or C ), d, E, and F. There are LED displays with more segments specially designed to display a wide range of alphanumeric and punctuation symbols. There are also dot-matrix displays which can display any pattern of dots. One way to make it easy to use 7-segment displays to show numbers is by using a decoder/driver chip such as the 4511 shown in Figure 37. It takes BCD input as four binary bits, and outputs the proper combination of voltages on its output pins to light up a 7-segment display to show the digit specified by the binary coded input. Additionally, the BLANK input (pin 4) will blank the display when it is driven low (to ground) regardless of the input. Normally it is held high. Pin 3, labelled TEST, lights all the segments at once when driven low. For normal operation it is connected to high voltage. The STORE input (pin 5) causes the decoder to hold on to the last value it saw, and continuously display that value. Otherwise the display changes as the input bits change. 48 [1.3.1; 07-OCT-2009]

49 3 DIGITAL CIRCUITS Electronics Merit Badge Handout # Vcc blank test D C B A 4511 store Gnd 8 a 13 b 12 c 11 d 10 e 9 f 15 g 14 Figure 37: Schematic Symbol for 4511 BCD-to-7-Segment LED Driver 3.6 Example Circuit: LED Sequencer This circuit would be acceptable as a project for requirement #4(b). Figure 38 shows an example using a decade counter and BCD-to-decimal decoder. Every time the clock pulse is triggered by pressing button S 1 the next light in the row lights up. Switch S 2 controls the direction the lights will flash. Pressing S 3 resets the counter back to zero again. +9V S 1 +9V +9V R 1 10k S 2 C 1 10µF 10 U/D Vcc 15 Clk +9V Gnd D 2 C 14 B 11 A 6 Preset 1 Reset 9 CI D C B A 16 Vcc 4028 Gnd 8 Q9 5 Q8 9 Q7 4 Q6 7 Q5 6 Q4 1 Q3 15 Q2 2 Q1 14 Q0 3 R 2 R 11D1 D 10 Figure 38: Example Circuit: LED Sequencer One interesting modification for this circuit is to remove the manual counter button (everything connected to pin 15), and replace it with a [1.3.1; 07-OCT-2009]

50 Electronics Merit Badge Handout #2 3.7 Example: Binary Counter timer circuit like the one shown in the free running counter project on page 13. This would make the LEDs light up in sequence on their own, at the speed the 555 timer circuit was set for. 3.7 Example: Binary Counter This circuit would be acceptable as a project for requirement #4(b). By simply taking the previous circuit and removing the BCD-to-decimal decoder, we can attach four LEDs to the raw binary output from the decade counter chip, and see our circuit count from 0 9 in binary. Figure 39 shows the modified circuit. This is a simpler circuit to construct because it elim- +9V +9V S 2 +9V U/D Vcc 4510 D 2 C 14 B 11 A 6 +9V S 1 R 1 10k C 1 10µF 15 Clk Preset 1 Reset 9 CI 5 Gnd 8 Figure 39: Example Circuit: Binary LED Counter inates a bunch of LEDs and a chip, but it is slightly more complex for the person using it since they have to interpret the output in binary. However, that would make it a good project to demonstrate how to count in binary. Note that we inserted inverter gates between the decade counter and the LEDs. We might have been able to get away with conecting the LEDs directly to the counter chip, but it is better to use an inverter or buffer to drive higher-current parts such as LEDs since most logic chips aren t designed to supply much current in their outputs. Since a typical inverter chip such as the 4049 actually comes with six gates on a chip (see Figure 21 on page 35), and since we already had two such gates employed to debounce the pushbutton, we used the remaining four inverter gates to drive the LEDs here. Since they do invert the signals, 50 [1.3.1; 07-OCT-2009]

51 3 DIGITAL CIRCUITS Electronics Merit Badge Handout #2 though, notice we turned around the LEDs compared to the previous version of the circuit, so they will light up when the binary counter output emits a 1 (since the inverter would then output a 0, which connects that end of the LED to ground, completing the current path to light it up). If we didn t already have a 4049 inverter chip in play, we could have left the LEDs turned their original direction (and connected to ground instead of +9 V), by using a buffer chip like the 4050 instead of the inverter shown here. 3.8 Example: Decimal Counter This circuit would be acceptable as a project for requirement #4(b). We can modify our design further, and make things really easy for the person using the circuit, by displaying an actual decimal digit as we count. Instead of any individual LEDs at all, we could just use a single 7-segment digit LED. As we already illustrated in Figure 36 (see page 48), a 7-segment digit consists of seven 8 long, narrow LED line segments arranged like the digit 8. One lead of each of these LED segments are connected together as the common pin of the display, and the other ends are individually connected to the circuit. This way, we can light up any combination of segments we need to, in order to form each digit. Figuring out which combination of segments should light up to make the digit represented by the four-bit BCD output from the decade counter is the job of the BCD-to-7-segment-LED-display driver chip. In our circuit we ll use the 4511 chip described on page 48. Since the 4511 chip sends positive voltage signals ( 1 ) for a segment which needs to be on, and ground ( 0 ) for those which need to be off, we will connect these outputs to the LED segments in the same way we did for the original LED sequencer circuit (Figure 38): the output goes through a current-limiting resistor to the positive side (the anode ) of the LED, and then all the negative side (the cathodes ) of the LED segments are connected to ground. This means we need to select a common cathode LED digit display when buying our parts. (These displays come in either common anode or common cathode versions.) Figure 40 shows the new circuit. 8 Or sometimes 8, if the LED includes a decimal point, or even 9, if it contains two decimal points. 51 [1.3.1; 07-OCT-2009]

52 Electronics Merit Badge Handout #2 3.9 Example: Free-Running Decimal Counter +9V S 1 +9V +9V R 1 10k S 2 C 1 10µF 10 U/D Vcc 15 Clk +9V Gnd D 2 C 14 B 11 A 6 Preset 1 Reset 9 CI Vcc blank test D C B A 4511 store Gnd 8 a 13 b 12 c 11 d 10 e 9 f 15 g 14 LED Figure 40: Example Circuit: Decimal Counter with Digital Output 3.9 Example: Free-Running Decimal Counter This circuit would be acceptable as a project for requirement #4(b). If we get tired of pushing the button to make any of the above circuits advance to the next number in their sequences, we can remove the button and debouncing circuit from pin 15 of the 4510 chip, and replace all of that with slow-running oscillator. Back on page 13 we saw a 555 timer chip arranged in this fashion, set up to flash an LED. Instead of driving an LED, we ll just feed the square wave output of the 555 oscillator circuit directly to pin 15, making the counter advance at the speed of the 555. Adjusting R 1, R 2, and C 1 will adjust the speed at which this will work. Handout #1 gave the details for calculating these values, but for one example, if we used R 1 = 1MΩ, R 2 = 1MΩ, and C 1 = 1µF, the counter will advance about once every other second. See Figure 41 for the replacement of the button with the timer circuit. Note that this modification can be made to any of the digital circuits above (sequencer, binary counter, and digital display counter). 52 [1.3.1; 07-OCT-2009]

53 4 TEST EQUIPMENT Electronics Merit Badge Handout #2 +9V +9V +9V 16 R 1 R 2 C Reset Vcc Discg Ctrl 5 Th +9V 555 Out 3 Trig Gnd 1 S 2 C µF 10 U/D 15 Clk Vcc 4510 Gnd D 2 C 14 B 11 A 6 Preset 1 Reset 9 CI 5 8 Figure 41: Example Circuit: Free-Running Counter Modification 4 Test Equipment To satisfy requirement #5(b), you need to be able to tell how three types of test equipment operate, and in general explain why test equipment is needed. Some additional kinds of test equipment relevant to the circuits we discussed today include the following. 4.1 Signal Generator A signal generator is essentially a fancy kind of oscillator which generates a waveform. Many signal generators offer a selection of wave shapes such as sine, square, ramp (or sawtooth), and triangle waves. They also allow you to adjust the amplitude (signal strength) and frequency of the signal. The signal generated by this device can then be input into an audio circuit such as a transmitter or amplifier, and an oscilloscope will show what happens to that waveform at various parts of the circuit you re testing. In this way, if the signal is being distorted by a problem in the circuit, you can track down the source of the trouble. 4.2 Logic Probe This is a very simple piece of test equipment, often built into a package resembling a large pen, as you can see in Figure 42. When the tip of the 53 [1.3.1; 07-OCT-2009]

54 Electronics Merit Badge Handout #2 4.3 Breakout Box probe is placed in contact with a signal source in a digital circuit, a light will light (or a tone will sound) to indicate whether the signal is high (indicating true or 1) or low (indicating false or 0). Even a device as simple as this can be very helpful in tracking down problems in digital circuits. 4.3 Breakout Box When diagnosing problems with serial or data connections between computers or computer-related components, it can be helpful use a breakout box. This device, pictured in Figure 43, is inserted into the cabling between them. It contains several LEDs which show the high and low digital signal levels on the cable wires. It also allows you to block some wires or divert their signals to other pins or even through other test equipment. 4.4 Logic Analyzer A logic analyzer is a much more sophisticated and expensive than a logic probe, but it is also for examining logic signals and determining whether they represent 1s or 0s. However, a logic analyzer examines several signal lines at the same time, and is able to perform complex tests. For example, it could test that a set of signals follow the correct sequence of signals over a period of time, or in response to certain input patterns. 54 [1.3.1; 07-OCT-2009]

55 4 TEST EQUIPMENT Electronics Merit Badge Handout #2 Figure 42: Logic Probe Figure 43: Breakout Box 55 [1.3.1; 07-OCT-2009]