Electronics Merit Badge Class Handout #1

Transcription

1 Electronics Merit Badge Class Handout #1 Counselor: Steve Willoughby Troop 547, Sunset Trail District, Cascade Pacific Council Phone: ; October 8, 2009 This handout contains notes from the electronics theory we discussed in our session today, plus extra material you may wish to use for reference later on, including schematic symbols, unit tables, part markings, and formulas. 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 #1 If you enjoy the Electronics merit badge, you may also be interested in these others: Electricity Radio Computers I am also a counselor for these merit badges, and would be happy to help with them. Colophon and Acknowledgements Text and schematic diagrams typeset using TEX, supplemented with pic, circuit macros and the m4 macro processor, using the Computer Modern typeface family at 11 pt. Urgent Mission (Benjamin Franklin) cartoon c Randal Munroe. Available online from Licensed under Creative Commons Attribution- NonCommercial 2.5 License; used with permission. Sine and rippled sinewave graphs from Wiki Commons, also licensed under Creative Commons terms and conditions. All other 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 c 2009 SLW 2 [1.6.7; 29-SEP-2009]

3 CONTENTS Electronics Merit Badge Handout #1 Contents 1 Course Outline 5 2 Hazards and Safety Precautions 6 3 Electrical Principles Charge Conductors & Insulators Current DC and AC Current Series and Parallel Circuits Ohm s Law Schematic Diagrams Connections Power Connections Ground Component Labels Switch Pushbutton Battery Resistor Variable Resistor Capacitor Diode LED Transistor Loudspeaker Fuse Circuit Breaker Transformer Integrated Circuit (IC) Example Circuit Schematics Regulated DC Power Supply Delay-Off LED Circuit One-Shot Flashing LED Constantly Flashing LED c 2009 SLW 3 [1.6.7; 29-SEP-2009]

4 Electronics Merit Badge Handout #1 LIST OF FIGURES 6 Test Equipment Ammeter Voltmeter Ohmmeter Multimeter Oscilloscope Handy References Base Units SI Prefixes Resistor Color Codes Assignments For Next Class 35 List of Figures 1 Comparison of AC and DC Waveforms Series Circuit Example Parallel Circuit Example Schematic Drawing Lines Showing No Connection Schematic Drawing Lines Showing a Connection Example Circuit: Regulated DC Power Supply Full AC Sine and Rectified (Rippled DC) Waves Example Circuit: Delay-Off LED Example Circuit: One-Shot Flashing LED Monostable Circuit Timing Diagram Example Circuit: Constantly Flashing LED Astable Circuit Timing Diagram Ammeter Test Connections Voltmeter Test Connections Ohmmeter Test Connections Multimeter (photograph) List of Tables 1 Base Units SI Prefixes and Their Meanings Resistor Color Codes c 2009 SLW 4 [1.6.7; 29-SEP-2009]

5 1 COURSE OUTLINE Electronics Merit Badge Handout #1 1 Course Outline Our coverage of the Electronics merit badge will follow this approximate schedule. Corresponding requrement numbers are shown in brackets, where applicable. Session #1 Safety Protocols [1] Basic Electrical and Electronics Principles [2, 4, 5] Test Equipment [6b] Ohm s Law [6a] Components and Their Schematic Symbols [2] Session #2 Audio Circuits [4c] Control Circuits [4a] Digital Circuits [4b] Session #3 Printed Circuit Boards [3c] Soldering Techniques [3a, 3b] Project Construction [4] Session #4 Project Construction [4] Careers in Electronics [7] You should expect to spend some time between class sessions doing some research on your own, so we can spend class time discussing what you found and passing you off on the requirements. c 2009 SLW 5 [1.6.7; 29-SEP-2009]

6 Electronics Merit Badge Handout #1 2 Hazards and Safety Precautions You need to know everything in this section. Be sure you understand all the safety rules before you start working with electronic circuits. Electric shock: Never work on live circuits. Always check before touching. Don t work on high voltage circuits at all without proper supervision. Ensure power can t be turned on while you re working. Dangerous chemicals: Use care with PC board etchants and other hazards. Use in ventilated area with adequate spill protection and cleanup supplies. Know all precautions and spill/exposure/poison control procedures for each substance before using. Use only with proper supervision. Always wear protective gear! Lead: Solder in ventilated area, with eye protection. Wash hands thoroughly before touching food or drink. Select lead-free solder and components when possible. Sharp metal: Wear eye protection and watch for flying wire clippings when trimming. High temperatures: Be responsible for where your hot soldering iron is, and what it touches, the entire time it is hot. Watch for trip hazards with your power cord don t add a flying 700 F iron on top of a fall! Wear eye protection. Beware of splattering solder/flux. Be prepared to administer first aid for burns. Amps vs. Volts: Don t be fooled thinking anything with a low voltage is completely harmless. In fact, what will generally injure or kill you is the number of amps (amount of current) flowing through you, if it manages to make contact. On the other hand, higher voltages are like having more pressure driving the current, and will allow the power to penetrate through insulation (such as your skin or other protection) if it s high enough. So you really need to watch for both. 12 volts at 10 amps is a world apart from 12 volts at.001 amps. c 2009 SLW 6 [1.6.7; 29-SEP-2009]

7 3 ELECTRICAL PRINCIPLES Electronics Merit Badge Handout #1 3 Electrical Principles In order to get the merit badge, you do not need to know this information per se, but reading through this information once or twice will give you some important concepts and context to help you understand everything else. You will need to know what current, voltage, power, capacitance and resistance are as general concepts, and the units of measurement used for each (amps, volts, watts, farads and ohms). 3.1 Charge Build-up of extra electrons (negative charge) or missing electrons (positive charge) on some material. Measured in coulombs (C). 1 C is the charge of electrons (that s 6,240,000,000,000,000,000). Now wait a minute, I hear you cry, The word positive sounds more like this is where stuff is and the word negative sounds more like where there s a lack of stuff... after all, a vacuum is negative pressure and so forth, right? Doesn t this seem backwards? Good question. It turns out that back when Benjamin Franklin was experimenting with electricity around two centuries ago, he discovered that a static electrical charge was built up (a positive charge ) on one material, and a lack of charge (or negative charge ) on the other. However, he didn t know the nature of what caused the charge (electrons had not been discovered yet), so it was impossible to say for sure which side was really the positive and which was the negative. So he just picked one and called it positive, assuming it was where the electricity was building up. Unfortunately, we discovered a century later that what we d spent all the intervening decades calling negative was in fact where the charge was, not the other way around. Now we had a dilemma. Either we rewrite all the textbooks, journals, books, papers, etc., change around all the symbols, and everything, and cause a lot of confusion, or just say that electrons are negatively charged particles and understand that contrary to our previous assumptions, the charge really flows from negative to positive. We chose the latter course, which means we didn t have to upset everything already established at that point, but it also introduces an annoyingly confusing situation where the terminology and symbols are still backwards from what s happening at the subatomic level. You have a choice, then, about which way you want to visualize the flow of current negative-to-positive or vice versa. Some people have a specific c 2009 SLW 7 [1.6.7; 29-SEP-2009]

8 Electronics Merit Badge Handout #1 3.2 Conductors & Insulators need to care about where the electrons themselves are going, so they use the negative-to-positive (called electron flow ) model. The rest of us, who are more concerned with how the circuit functions as the current flows (in whatever direction) through it, don t need to consider that level of detail, and stick to the traditional positive-to-negative model (called conventional flow ) because it feels more intuitive and lets all the established terminology and circuit diagram symbols still make sense. We will assume a positive-to-negative flow for the rest of this course. 3.2 Conductors & Insulators Some kinds of atoms, such as copper, gold, and iron, allow their electrons to jump easily from atom to atom, creating a flow or current of these particles. These are called conductors. Other atoms hold on to their electrons more tightly and do not allow them to flow around. These are insulators. Some, like carbon, will conduct but not nearly as well, and will only allow part of the current through (or in other words, they provide resistance to the flow of electrons in a manner similar to how a heavy filter might resist the flow of water in a pipe). Still other elements, such as silicon, will conduct under some circumstances and not under others. These are called semiconductors and by controlling when they will or won t conduct (and varying how much they will conduct) we can construct interesting and complex circuits from AM radios to supercomputers. c 2009 SLW 8 [1.6.7; 29-SEP-2009]

9 3 ELECTRICAL PRINCIPLES Electronics Merit Badge Handout #1 3.3 Current The flow of electrons between atoms of conductive materials (e.g., copper, aluminum, gold) is called current and is represented in formulas by the symbol I. The amount of current flowing past a given point is measured in coulombs per second (C/s) which has its own specific unit: amperes (A). This flow of electrons is driven by the electromotive force (EMF) trying to cancel the difference in charge between positive and negative. The greater the difference in charge, the stronger the force driving the current. This force, represented in formulas by the symbol E, is measured in volts (V). Resistance against the flow of current is represented by the symbol R and is measured in ohms (Ω). Power, or the rate at which energy is used, is represented by the symbol P and is measured in terms of joules per second (J/s), which has its own specific unit name: watts (W). I will give a bonus prize next time to anyone who can tell me why the symbol for current is the letter I. 3.4 DC and AC Current Direct Current (DC) is the kind of power you get from a battery or DC power adapter. The output is a steady, constant voltage either positive or negative with respect to a common ground reference. Alternating Current (AC) is what a typical generator produces, and is the kind of current supplied to our home wall outlets. The voltage supplied by an AC source is constantly fluctuating back and forth between positive and negative. Household current, for example, alternates at a speed of 60 Hz, or in other words, it makes a full cycle from 0 V up to +170 V, back through 0 to 170 V, and back up to 0 again, 60 times per second. This speed, or the number of times the current makes a complete cycle per second, is called the frequency of the signal. If we were to plot the voltage output over time on a graph, we would see a sine wave like the one shown in Figure 1. Compare the AC waveform on the left with the steady voltage from a DC power source on the right: Note that the voltage measured on an AC power line is the average voltage (also called the root mean square or RMS voltage). As the voltage increases and decreases through each cycle, it will actually reach a much higher peak for a brief instant before coming back down. For a 120 V household circuit, for example, the current actually cycles from a peak of approximately +170 V, through 0, and down to a peak of c 2009 SLW 9 [1.6.7; 29-SEP-2009]

10 Electronics Merit Badge Handout #1 3.5 Series and Parallel Circuits Figure 1: Comparison of AC and DC Waveforms 170 V, and back up. The RMS voltage, what you d actually read on a voltmeter, is times this peak value: V RMS = V Peak So, if a power line s peak output hits ±170 V, the measured RMS voltage will be or about 120 V. 3.5 Series and Parallel Circuits When a set of components are wired in a single line, where the current must flow through one, then on through the next, and so on, this is called a series circuit. For example, in the circuit in Figure 2, the LED will only light up if all of the switches are closed, since power will need to flow throuch each of them to complete the circuit. + Figure 2: Series Circuit Example c 2009 SLW 10 [1.6.7; 29-SEP-2009]

11 3 ELECTRICAL PRINCIPLES Electronics Merit Badge Handout #1 A lot of Christmas light strings are wired in series, too. That s why when one bulb burns out or is removed, they all go out. The power needs to flow through all of them to have a complete circuit. On the other hand, a parallel circuit (like the one shown in Figure 3) is one where the components are independently wired as parallel paths so that the current only needs to flow through one of them to have a complete circuit. In the following circuit, the LED will light up if any of the switches are closed. + Figure 3: Parallel Circuit Example 3.6 Ohm s Law To get the merit badge, you need to know the basic formulas described here. (Note that if you know P = IE and E = IR you can derive the others from them using basic algebra.) To pass requirement #5(a), your counselor will give you a simple problem involving current, voltage, and resistance. You will need to demonstrate that you know the correct formula(s) to use, and then use them to get the correct answer to the problem. Ohm s law 1 tells us the relationship between current (I), resistance (R), electromotive force or voltage (E), and power (P): P = IE; I = P E ; E = P I E = IR; I = E R ; R = E I 1 Actually, this is a combination of Ohm s Law (E = IR) and one of Joule s laws (P = I 2 R). c 2009 SLW 11 [1.6.7; 29-SEP-2009]

12 Electronics Merit Badge Handout #1 It is easier to remember these formulas by picturing the values E, I, and R (or P, I, and E) arranged like the diagrams on the right. If you cover up the variable you need to find, the position of the E I R P I E other two tells you what math you need to apply to obtain it. For example, if you need voltage (E) but know resistance (R) and current (I), then use the EIR chart, covering up the E. This leaves IR next to each other, which means to multiply I R. Or, if you know power (P) and voltage (E) but need to know current (I), then you use the PIE chart, covering up I. This leaves the P over the E, meaning divide P E. 4 Schematic Diagrams To satisfy requirements #2 and #4, you need to know a few basic electronic components and their schematic symbols, including resistors, capacitors, transistors, and integrated circuits. Schematic diagrams show the components of a circuit using standard symbols to represent each one, and the connections between the components. They are drawn in a way which communicates the function of the circuit, not necessarily matching the physical appearance or arrangement of the parts involved. 4.1 Connections Connections between components are drawn as solid lines between them. If two lines cross over each other, they are not assumed to be connected together (see Figure 4). Figure 4: Schematic Drawing Lines Showing No Connection It s generally preferable to show unconnected cross-overs with a break or hop at the intersection instead of simply letting the lines cross, because that way it is more clear that they are not actually connected. c 2009 SLW 12 [1.6.7; 29-SEP-2009]

13 4 SCHEMATIC DIAGRAMS Electronics Merit Badge Handout #1 If two or more wires are to be connected, there will be a small dot at their intersection to indicate this (see Figure 5). Figure 5: Schematic Drawing Lines Showing a Connection 4.2 Power Connections When connecting a point in the circuit to power, you could draw a solid line from the power jack, battery terminal, +5V etc., to every single part, but a better alternative is to use a power connection symbol. This simplifies the schematic diagram by hiding the power bus, with the implication that all these power connection points are wired together to the power supply. Write the voltage being connected to above the symbol, especially if multiple voltages are provided to the circuit. 4.3 Ground A power connection to the ground voltage (the reference point from which all other voltages are measured in simple circuits, this is usually the negative side of the power supply or negative battery terminal) is shown by drawing a wire down, terminating in the symbol shown here on the left. The symbol on the right (resembling a sideways E ) indicates a connection to the chassis (the box containing the circuit) itself. 4.4 Component Labels When drawing each component in a schematic, it should be labelled with a part number and/or value. If part numbers alone are used, a separate parts list will show all the parts R15 150Ω required to build the circuit, all their values and other specifications, and which part numbers they represent. c 2009 SLW 13 [1.6.7; 29-SEP-2009]

14 Electronics Merit Badge Handout #1 4.5 Switch Part numbers typically consist of a prefix code indicating the type of component ( R for resistor, C for capacitor, Q for transistor, IC or U for integrated circuit, D for diode, etc.) followed by a number counting from 1 sequentially so each part has a unique ID. (For example, the resistors in a circuit would be named R 1, R 2, and so forth.) 4.5 Switch Switches can either interrupt the flow of current, as a drawbridge can open to interrupt the flow of traffic, or can be used to select one of several alternative paths for the current to take. The simplest style of switch is shown here. It is called a Single-Pole, Single-Throw (or simply SPST ) switch. This is a basic on/off current interrupt device. DPST (Double-Pole, Single-Throw) switches are the same, except they have two sets of switch contacts tied together, so that two different circuit paths are switched on or off at the same time. The SPDT (Single-Pole, Double-Throw) and DPDT (Double-Pole, Double Throw) switches select between two possible paths for the current to take. 4.6 Pushbutton Buttons are just like switches except the physical action required to open or close their contacts is to push a button instead of flipping a switch. Typically they are momentary action the contacts close when you re holding down the button but spring back to their open position when you let go. However, many other varieties of pushbutton are possible. They may come in SPST (as shown here), or SPDT (or possibly other combinations). An SPST pushbutton may even be normally open (so the contacts close while you push the button) or normally closed (so the contacts are closed and current flowing through the button normally, and pushing the button opens the contacts, interrupting the current while you hold the button down). A button may even be push-on-push-off, changing between open or closed state each time you press it. c 2009 SLW 14 [1.6.7; 29-SEP-2009]

15 4 SCHEMATIC DIAGRAMS Electronics Merit Badge Handout #1 4.7 Battery A battery provides a steady supply of DC current to the circuit through a chemical reaction (typically between an acid paste and two different kinds of metal, such as copper and zinc). The symbol for a power cell is a long line representing the positive terminal and a shorter line representing the negative terminal (note the similarity and + differences between the battery and capacitor symbols). A battery consists of multiple cells, so the basic cell symbol is repeated 2 3 times to form the symbol for a battery. I ll give a bonus prize next time to any scout who can name a pair of common metals used in rechargable batteries. 4.8 Resistor A resistor is a component which resists the flow of current through it. Use it to reduce voltage, to pull up or pull down data signals in digital circuits (as we ll see in the next session), and to limit current through LEDs. A resistor s value (how much resistance it provides to a circuit) is measured in ohms (Ω). Typical resistors range from a few ohms up to many thousands ( kilohms kω) or millions ( megohms MΩ). This is indicated on the part itself using color-coded stripes around the resistor body. (See page 34 for a chart indicating this color code.) Resistors are also rated in terms of the amount of power (in watts) they will handle. The greater the wattage rating, the larger the resistor will physically be. Resistors connected in series will together provide a total resistance which is the sum of each of their individual values. In other words, for n resistors in series: R total = R 1 + R 2 + R R n However, if they are arranged in parallel, the current flowing through them won t be evenly distributed (unless these are all equal in value), it gets just slightly more complicated. The total is, for n resistors all in parallel: 1 R total = 1 R R R R n c 2009 SLW 15 [1.6.7; 29-SEP-2009]

16 Electronics Merit Badge Handout #1 4.9 Variable Resistor 4.9 Variable Resistor A variable resistor, or potentiometer, consists of a fixed resistor (which has a value like a normal resistor) and a movable contact point which allows current to be passed from that point to either end of the fixed resistor. By moving the contact, the amount of resistance can be changed at any time. These can be arranged with knobs for adjusting easily at will (such as a volume control), or they can be miniature trimmer potentiometers (commonly called trim pots ) which are mounted on a circuit board, adjusted once with tools and left alone Capacitor A capacitor consists of a pair of metal plates separated by a thin insulator (called a dielectric ). When power is applied, a charge builds up on the plates, which is later released ( discharged ) when the incoming power is less than the charge on the plates. In the illustration shown here, the capacitor on the top is a common disc-type capacitor, while the one on the bottom is an electrolytic capacitor. Electrolytic capacitors can hold a higher amount of capacitance in a smaller space but are often polarized (so one terminal must be connected + to positive, the other to negative power). The polarization is marked on the schematic symbol and on the part itself. The amount of capacitance any given capacitor holds is rated in units of farads (F). One farad is so huge, it is unlikely you ll ever deal with a capacitor as large as even 1 F. Typical values used in electronic circuits range from as small as a trillionth of a farad ( picofarads pf) up to as large as a few millionths of a farad ( microfarads µf). Capacitors are used to smooth out ripple in power supply lines caused by rectifiers converting AC to DC, or introduced by fast-switching components such as digital logic gates. They pass AC signals (including audio or radio signals) but block DC. This may be used to separate the AC and DC parts of a circuit, or to filter out noise created by one part of a circuit so it doesn t interfere with the rest. In very small circuits, a capacitor might be used as a very short-term, tiny battery to keep essential parts of a circuit (such as data in a memory c 2009 SLW 16 [1.6.7; 29-SEP-2009]

17 4 SCHEMATIC DIAGRAMS Electronics Merit Badge Handout #1 chip) alive while power is switched off. They can be used to make very strong, short pulses of power for applications where short bursts are needed (e.g., some kinds of weapons and device control circuits). If capacitors are arranged in parallel, the total capacitance provided by the set is simply the sum of their individual values. In other words, for n capacitors in parallel: C total = C 1 + C 2 + C C n By contrast, if you arrange n capacitors in series, the total is: 1 C total = 1 C C C C n Note that this is the opposite of the formulas used for resistors in series and parallel. Capacitor values are marked in printed text (usually) or colored dots or stripes (rarely seen in the kinds of applications we re dealing with). Larger capacitors are usually explicitly marked, e.g., 4700 µf, 16V. Smaller capacitors are marked in picofarads using a system similar to resistors, but with printed digits instead of colored stripes. For example, a small capacitor marked 334 would be pf, or 0.33 µf Diode A diode is like a check valve for electric current. It allows current to flow in one direction (from anode to cathode, or in the direction of the arrow if you consider the direction to be from positive to negative), but not in the other. It presents very little resistance in the forward direction, so if connected directly between positive and negative power, would effectively be a short circuit. Diodes are marked with a colored band on the cathode (negative terminal). The anode is sometimes labelled A and the cathode is labelled K in diagrams, on circuit boards, and sometimes on the components themselves. 2 Or 330 nf, although the use of nanofarads as a unit is more common in Europe than in North America, where we usually just use microfarads and picofarads. c 2009 SLW 17 [1.6.7; 29-SEP-2009]

18 Electronics Merit Badge Handout # LED 4.12 LED Light-Emitting Diodes, or LEDs for short, are essentially just like other diodes, except that when current flows through them in the forward direction, they light up. LEDs are available in various sizes, brightness, and colors, including infrared and laser-emitting varieties. Compared to regular incandescent lamps, LEDs consume very little power, turn on or off instantly, and last a very long time (usually outliving the circuit they are connected to). They are sensitive to being overloaded, however, and their maximum current rating must never be exceeded. The cathode of an LED is usually marked by a shorter lead and/or a flattened spot along the circumference of the round LED base. The schematic symbol may (or may not) include a circle around the diode, but the arrows pointing away from the part indicate that it is a light-emitting kind of diode as opposed to a regular diode. (Note that there is also a light-sensitive diode, with the arrows pointing in toward the part, which is a completely different component.) Since it is a diode, an LED would appear as a short circuit unless something (usually a resistor) is placed in series with it to limit the current flowing through it. LEDs are current-driven devices. They can operate at any voltage within their maximum rated range, but the light output depends on the amount of current (amps) flowing through it, up to (but never in excess of) the maximum rating. A typical 5 mm LED consumes about 20 ma (or 0.02 A). To properly size the resistor to pair with an LED, you must know: the voltage supplied from the circuit (represented by the symbol V CC or sometimes V DD ), the voltage drop provided by the LED itself when forwardbiased (i.e., when current flows in the forward direction, how many volts are lost going through the LED) (V F ), and the desired current to run through the LED (I F ). You can get V F and I F from the LED data sheet available from the manufacturer. Knowing those values, and the fact that the voltage which will be operating here is the supply voltage minus the amount dropped by the LED, V CC V F, it becomes a simple matter of applying Ohm s Law (see page 11) to determine the resistance needed, recalling that R = E I : R = V CC V F I F c 2009 SLW 18 [1.6.7; 29-SEP-2009]

19 4 SCHEMATIC DIAGRAMS Electronics Merit Badge Handout #1 and then rounding up to the next standard resistor value you can actually buy. To figure out the wattage of the resistor, apply Ohm s Law again. Since P = I E: P = I F (V CC V F ) and allow a little room (i.e., don t size the resistor so it must work at full rated capacity). For example, if we have an LED with I F = 20mA, which gives a voltage drop V F = 1.2V, in a circuit with V CC = +5V, then the resistor is: R = V CC V F I F = = = 190Ω We can use the standard value of 220 Ω without difficulty. The power dissipated by the resistor in this circuit will be: P = I F (V CC V F ) = 0.02 (5 1.2) = = 0.076W which is pretty small. Resistors normally come off the shelf as small as W (1/8 W), so we can use that and be well within our limits here Transistor Transistors are similar to diodes, except that they have three pins, called the base (B), collector (C), and emitter (E). Current normally flows between the collector and emitter, much the same way as it does between the anode and cathode of a diode. In this case, however, we can control how much will flow by applying a small current to the base of the transistor. B C E NPN B C E PNP This will allow the transistor to function as a solid-state switch if we saturate the transistor by giving it enough current at the base to turn c 2009 SLW 19 [1.6.7; 29-SEP-2009]

20 Electronics Merit Badge Handout # Loudspeaker fully on, or removing that current to turn it fully off. Most digital logic gates work like this. Transistors also work quite well as signal amplifiers, since a small change in current at the base is enough to control the flow of a much larger current between the collector and emitter, making the resulting output signal much greater but still having the same waveform shape. Transistors may be marked with B, C, and E next to their leads, or you may need to look up the lead assignments from the part s datasheet. Beware, not all transistors assign the same pins in the same locations, so always check to be sure. There are many types of transistors available. The two shown here are bipolar junction transistors which are the most common basic type. Another common type is the field effect transistor (FET). Bipolar junction transistors come in two basic types: NPN and PNP. These differ in the direction current flows through them: NPN is for current flowing (positive-to-negative) from the collector to emitter; PNP flows in the opposite direction. Transistors are sensitive to heat and static electricity. Use caution (and a heat sink) when handling them and soldering them onto a PC board Loudspeaker Loudspeakers use an electromagentic coil to move a membrane back and forth as current is run through the coil. This allows a circuit to turn an electronic waveform into audible sound vibrations. A microphone is, at least in general concept, the opposite of this it takes sound wave vibrations and converts them to an electrical signal. In fact, a speaker can be used as a microphone (although it won t sound as good as a microphone specially built for that purpose), and vice versa Fuse Fuses are used to protect circuits from being overloaded. If too much current flows through a wire, PC board trace, or component, that part will heat up. If enough excess curent is passed, enough heat may be produced to catch the parts on fire or become permanently damaged. c 2009 SLW 20 [1.6.7; 29-SEP-2009]

21 4 SCHEMATIC DIAGRAMS Electronics Merit Badge Handout #1 A fuse is a thin wire designed to self-destruct at a particular current level, with the idea that it will do so before anything else in the circuit sustains damage. The fuse will then need to be replaced Circuit Breaker A circuit breaker performs the same function as a fuse, but rather than destroying itself when overloaded, it simply acts as a switch which automatically opens. Once the overload condition is corrected, the switch can then be reset and normal operation resumes. There are several types of circuit breaker, including current overload protection breakers and ground-fault protection breakers Transformer Transformers are one way to pass power and/or signals (such as audio waveforms) between parts of a circuit without a direct wired connection between those two parts. The connection is made magnetically. The incoming power is run through the transformer s primary coil which becomes an electromagnet, creating a magnetic field around the transformer s core. This core may be an iron bar, or even just air. On the other side of the core is a secondary coil connected to the other side of the circuit. Although not electically wired to each other, the secondary coil will convert the magnetic field cutting through it into an electic current which will flow into the attached circuit. This only works as long as the magnetic field moves across the coil, either by the voltage changing (making the field expand and contract) or by physically moving the coil or magnet (as a generator does). In the case of a transformer, this means only AC current, which is constantly changing voltage and switching direction, will activate the transformer and be picked up in the secondary coil. If the secondary coil is the same size (same number of windings of wire) as the primary, the voltage output there will be the same as that input on the primary side. However, if the secondary has, for example, 1/4 the number of windings, then only a fourth of the magnetic force will be picked up and converted to voltage, so the output voltage will the 1/4 the input c 2009 SLW 21 [1.6.7; 29-SEP-2009]

22 Electronics Merit Badge Handout # Integrated Circuit (IC) voltage. If 120 V AC is applied on the primary, 30 V AC would be output at the secondary. It works the other way, as well. If the secondary has twice the number of windings as the primary, the output voltage would be doubled. According to Ohm s Law, if the voltage changes, the current flow will change with it, in the opposite direction. If you double the voltage, then, you ll get half the current Integrated Circuit (IC) Integrated circuits (also called ICs or simply chips ) are useful mini-circuits which are reduced until their components are of microscopic size, and are formed on silicon chips and packaged into convenient cases with multiple pins to connect them into a larger circuit. This way, a common complex (and oftenrepeated) part of a circuit becomes itself just an individual component (albeit a very complex one). ICs can provide anything from logic gates, to clock/timer circuitry, to radio receivers, audio amplifiers, or even full-blown computers on a chip which can provide complex control to a larger circuit. The IC shown here is the LM555, which is a popular timer, and a good chip for beginners to experiment with. It has eight pins. Two of them Reset Vcc Discg Th Trig Gnd 1 8 Ctrl 5 Out 3 (pins 1 and 8) supply power to the circuitry within the IC, and are just connected to the circuit s power. The remaining pins allow us access to any relevant inputs, outputs, and control functions our circuit needs to connect to. ICs are identified by a part number (like LM555 for this one) stamped onto the package. A notch or small hole indicates the location of pin #1, with the remaining pins being numbered counter-clockwise around the package from there. ICs are extremely sensitive to static electricity. Never handle them without taking proper precautions. They are also very heat-sensitive. You should solder a socket onto your circuit board, and then insert the chip into the socket. If you must solder a chip directly to a board, be extremely careful and use a heat sink. c 2009 SLW 22 [1.6.7; 29-SEP-2009]

23 5 EXAMPLE CIRCUIT SCHEMATICS Electronics Merit Badge Handout #1 5 Example Circuit Schematics To satisfy requirement #2, you need to draw a schematic diagram including resistors, capacitors, and either transistors or integrated circuits. All of the schematic diagrams in this section would be acceptable for you to copy and draw yourself. Be sure to label the parts and be able to describe what each part does in the circuit. This is explained in the text in this section for each example circuit. These are some example circuits to give you a preview of what we ll be discussing next time, as well as to illustrate the concepts we covered so far. Read up on the operation of these circuits, do a little research on them if you like, and we ll pick up from here in the next session. 5.1 Regulated DC Power Supply This is the example circuit we discussed in class. It is a simple regulated +5 V DC power supply such as you might want to use for your workbench. The schematic is shown in Figure V AC Input F 1 S 1 T 1 D 1 D µF C 1 In 1 C 2.33µF U Gnd 2 Out 3 C 3.1µF +5V R 1 220Ω D 5 Figure 6: Example Circuit: Regulated DC Power Supply Caution: This circuit runs on 120 V AC power, and is included here as an example of how power supplies are typically constructed and to illustrate the concepts discussed in class. Do not build a circuit involving high voltages such as this without qualified supervision. These voltages can be hazardous or fatal. The input of the circuit (on the left of the schematic diagram) is connected to a 120 V AC plug. This power is routed through a fuse (F 1 ) and a switch (S 1 ) which we can use to turn on or off the power supply. Finally, the 120 V current passes through transformer T 1 which drops it to 9 V. c 2009 SLW 23 [1.6.7; 29-SEP-2009]

24 Electronics Merit Badge Handout #1 5.1 Regulated DC Power Supply The 9 volts are closer to what we need, but still AC at this point, so we have diodes D 1 D 4 arranged as a rectifier bridge which routes the positive and negative current to the positive and negative DC parts of our circuit, respectively, regardless of which direction the AC is flowing at any given moment. The AC power is converted to DC at this point, but since all we ve done is reflect the negative half of the sine curve to stay above the 0 V line. However, it still will drop to 0 V and back up to 9 V every 1/120th of a second. This ripple is unacceptable to sensitive electronic components, so we need to do something about it. The AC (between T 1 and D 1 D 4 ) and DC (between D 1 D 4 and C 1 ) waveforms we observe look like these shown in Figure 7. Figure 7: Full AC Sine and Rectified (Rippled DC) Waves We place a 4700 µf electrolytic capacitor 3 (C 1 ) across the power coming from the rectifier bridge. This will charge up as the current flows, and hold it until the incoming power becomes less than what s stored in the capacitor. C 1 will then discharge its stored energy to fill in until the next cycle repeats the process. This will smooth out the ripple and produce a nearly flat 9 V DC signal. From there, we go to U 1, which is a 7805 voltage regulator chip. This takes the input voltage and drops any excess voltage so that the output stays at exactly +5 V, no matter how the input voltage might fluctuate. It can t create power from nothing, though, so the input needs to be at least a couple of volts above 5 V at all times in order for U 1 to function properly. Capacitors C 2 and C 3 provide some additional filtering and are recommended to be used along with U 1. 3 The exact value isn t that important. I just happened to have one this size handy, and used it. c 2009 SLW 24 [1.6.7; 29-SEP-2009]

25 5 EXAMPLE CIRCUIT SCHEMATICS Electronics Merit Badge Handout #1 Finally, D 5 is an LED we can mount on the panel of the power supply chassis to indicate when the supply is turned on and producing power. Note R 1, a 220 Ω resistor limiting the current through D 5 to 20 ma. 5.2 Delay-Off LED Circuit This circuit (Figure 8) shows how we can use a capacitor to delay when an LED turns off. S 1 B 1 9V + R 1 22K Q 1 2N C 1 D 1 10µF R 2 470Ω Figure 8: Example Circuit: Delay-Off LED Transistor Q 1 controls current flowing to LED D 1. At first, there is no current at the base of Q 1, so it is off and no current flows through it, leaving D 1 off. When we press the pushbutton S 1, current flows into the base of Q 1, turning it on and lighting up D 1. At the same time, current flows into C 1, charging it up. When S 1 is released, the stored charge in C 1 is released, supplying enough current to the base of Q 1, keeping D 1 lit until C 1 discharges and Q 1 turns off again. 5.3 One-Shot Flashing LED This circuit uses the 555 timer IC, arranged as a one-shot circuit (also called a monostable multivibrator circuit). Its schematic is shown in Figure 9. c 2009 SLW 25 [1.6.7; 29-SEP-2009]

26 Electronics Merit Badge Handout #1 5.3 One-Shot Flashing LED +9V +9V +9V 10K R 1 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 9: Example Circuit: One-Shot Flashing LED When the trigger button is pushed momentarily, the LED will light up and stay lit for a certain length of time (even after the button is released). Most of the work to accomplish this takes place within the 555 timer IC. We supply a few key components and connections to control it. The oneshot timer cycle is started when the IC s trigger input (pin #2) transitions from +9 V to ground. Normally, a positive voltage through R 3 is provided to keep the trigger line held high. When button S 1 is pressed, it connects the trigger directly to ground, signalling the IC to begin a timer cycle. (This doesn t create a short circuit because R 3 is still providing a load between +9 V and ground.) The IC will then emit +9 V from its output pin (pin #3), which lights the LED (D 1 ). The resistor and capacitor R 1 and C 1 control the length of time which will pass before the timer cycle ends and the output drops to ground again, turning off the LED. The IC controls the charging and discharging of C 1 and uses the length of time it takes to accomplish that to control how long the timer cycle will last. The exact time the output will remain on (T in the timing diagram shown in Figure 10) is calculated as T = 1.1RC, where R is the value of resistor R 1 in ohms, and C is the value of capacitor C 1 in farads. So if we wanted the LED to remain on for 10 seconds, we could use a c 2009 SLW 26 [1.6.7; 29-SEP-2009]

27 5 EXAMPLE CIRCUIT SCHEMATICS Electronics Merit Badge Handout #1 Trigger Output T Figure 10: 555 Monostable Circuit Timing Diagram 900 kω resistor for R 1 and a 10 µf capacitor for C 1 : T = 1.1 R C = = 9.9 seconds 5.4 Constantly Flashing LED This is similar to the previous circuit, but this time we hook up the 555 IC in a different configuration. This makes it self-triggering, so it constantly cycles on and off. This is called an astable multivibrator or oscillator circuit. The waveform produced on pin #3 of the 555 chip will be a square wave. (See Figure 11 for the schematic diagram.) As in the previous circuit, the 555 chip cycles the output between on and off. However, instead of manually triggering each cycle, we connect the trigger input of the chip to keep triggering it again at the end of each cycle. In this example circuit, we use the output to drive an LED, which will flash on and off at the rate determined by R 1, R 2, and C 1. As we ll see below, instead of making an LED flash, we could connect this basic 555 circuit s output to any other circuit which required a square wave input. The output waveform is a square wave as shown in the timing diagram in Figure 12, but this time we have two resistors and a capacitor to control the timing. We can actually control the length of time the output is on, and the length of time it is off, so it is possible to make the LED flash for a split second, and then remain off for a longer period, or any combination of different timings. The length of time the output remains on, as a fraction of the overall cycle time, is called the duty cycle of the output. If the output is on 1 s c 2009 SLW 27 [1.6.7; 29-SEP-2009]

28 Electronics Merit Badge Handout #1 5.4 Constantly Flashing LED 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 11: Example Circuit: Constantly Flashing LED T 1 T 2 Figure 12: 555 Astable Circuit Timing Diagram c 2009 SLW 28 [1.6.7; 29-SEP-2009]

29 6 TEST EQUIPMENT Electronics Merit Badge Handout #1 and off 3 s, that would be a 25% duty cycle (the output is on 25% of the time). The time the output is on, T 1, is calculated based on the values of all three components: T 1 = 0.695(R 1 + R 2 )C where R 1 and R 2 are the values of reisitors R 1 and R 2 in ohms, respectively, and C is the capacitance of C 1 in farads. The time the output is off, T 2, is based only on the values of R 2 and C 1, and is simply T 2 = 0.695R 2 C. If we wanted the LED to flash about once every 2 seconds, we could use 1 MΩ resistors for both R 1 and R 2, and a 1 µf capacitor for C 1 : T 1 T 2 = (R 1 + R 2 ) C = ( ) = = 1.39 seconds = R 2 C = = = seconds The total cycle period T 1 + T 2 = seconds. If we took that same circuit and connected a speaker in place of the LED, we could make an audio oscillator (tone generator). For example, with values of 3.9 kω for R 1 and R 2, and 0.15 µf for C 1, the total cycle time would be sec. This means we would get about 819 cycles per second, or an 819 Hz audio tone (just a bit lower than A 5). 6 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. The test equipment we discussed included the following. (We will discuss more kinds of test equipment in the next session as well.) 6.1 Ammeter This is typically an amps mode of a multimeter or a standalone ammeter, which is wired in series with the circuit being tested (so the current flows c 2009 SLW 29 [1.6.7; 29-SEP-2009]

30 Electronics Merit Badge Handout #1 6.2 Voltmeter through the meter as part of its path between the power source and the circuit). Ammeter Load Figure 13: Ammeter Test Connections Be careful to ensure your meter is set for the correct range of amps before connecting it to the circuit! Exceeding the selected range (e.g., setting the meter to a 2mA range but putting 200mA into it) or to an incorrect mode (e.g., setting the meter to ohms when amps was needed) may result in permanent damage to the meter, circuit, and/or you. Another type of ammeter clips around one of the power wires and measures the current flowing through the wire (by induction) without needing to be hardwired into the circuit path itself. 6.2 Voltmeter This is typically a volts mode of a multimeter or a standalone voltmeter. This measures the difference in potential between two points, often between a point of interest and the circuit s ground. Load Voltmeter Figure 14: Voltmeter Test Connections Be careful to ensure your meter is set for the correct range of volts before connecting it to the circuit! (See also the warnings under ammeter above.) c 2009 SLW 30 [1.6.7; 29-SEP-2009]

31 6 TEST EQUIPMENT Electronics Merit Badge Handout #1 6.3 Ohmmeter This is typically an ohms mode of a multimeter or a standalone ohmmeter. This measures the resistance through a single component or a specific path through a circuit. To do this, the meter itself applies a small current through the part and measures how much makes it to the other end. This means you need to ensure external power isn t also applied to the circuit or you ll risk overloading the meter. Also be careful that the part in question can handle the meter s current being sent through it (usually not a problem, but worth noting). Load Ohmmeter Figure 15: Ohmmeter Test Connections An ohmmeter may be used to measure continuity (whether a solid conductor path exists between two points). For example, to test a trace on a PC board or wiring in a circuit to check for loose or broken connections, and to make sure the right components are connected to the right other ones. Some multimeters have a special continuity mode which gives an audio or visual indication that power is able to get through. With a regular ohmmeter, check that the meter reads near 0Ω through the path. Be careful to ensure your meter is set for the correct mode before connecting it to the circuit! (See also the warnings under ammeter above.) 6.4 Multimeter This is a digital multimeter (or DMM for short). Many varieties of these meters are available, and can be a good alternative to buying several separate meters. This particular one functions as a voltmeter (0.1 mv 750 V AC or 0.1 mv 1 kv DC), ohmmeter (0.1 Ω 20 MΩ), ammeter (0.1 µa 10 A DC or AC), continuity tester, and diode tester. 6.5 Oscilloscope This is a device we ll see more of in the upcoming sessions. It draws a realtime graph of a signal waveform. For example, it would let you visually see c 2009 SLW 31 [1.6.7; 29-SEP-2009]

32 Electronics Merit Badge Handout #1 6.5 Oscilloscope Figure 16: Multimeter c 2009 SLW 32 [1.6.7; 29-SEP-2009]