What are PROCESS COMPONENTS? Input Transducer Process component Output Transducer The input transducer circuits are connected to PROCESS COMPONENTS. These components control the action of the OUTPUT components depending on the signal received from the INPUT component. The process will do nothing until it receives information from the INPUT component. INPUT components alone cannot switch on OUTPUT components because they do not have a current driver to be able to operate any OUTPUT device. PROCESS components covered in the syllabus: 37
What is a Transistor? It is known as the bi-polar transistor and made from 3 layers of semiconductor material the transistor is an amplifier of current, a current driver, and also as an electronic switch operated by voltage. The semi-conductor material used is N-type and P-type. They are layered together as follows: Collector Each leg performs a different function and therefore is labelled accordingly: Base N P N The symbol for the Transistor: Emitter Collector Base Emitter How does it work? The collector is connected to the OUTPUT component and the Base is connected to the INPUT component. The Emitter emits electrons back to the battery and therefore is connected to the battery 0V or black wire. When a voltage of over 0.7 Volts is applied to the BASE through a limiting resistor, the Transistor connects Base Small Current Collector Emitter Large Current Collector to Emitter and switches ON. In so doing a LARGE current now flows from Collector to Emitter through the Transistor and turns ON the OUTPUT component. To do this only a tiny amount of current is applied at the BASE through a limiting resistor. 38
So to connect up an input component to a transistor, the circuit would look like this: OUTPUT GOES HERE +6 10K Collector 1K Base Limiting Resistor Emitter Power Supply INPUT CIRCUIT PROCESS The purpose of the limiting resistor is to limit (control) the amount of current flowing into the base. Without a 1K resistor too much current will flow between BASE and EMITTER and the transistor will fail. Any input sensing circuit can be connected to the transistor as described in the arrangement above. Transistors come in packages such as these, and you will need a key to decipher which legs are the base, collector and emitter and so on. RAPID electronics catalogues include a key. You can see a copy of this key on the next page. Inside a BC184. 39
Decoding Transistor Legs Identify from the catalogue which can type the transistor is packaged in for example TO92 Hold the transistor with the legs pointing towards you with the flat in the same place as on the diagram. Now you can find the C, B and E. The T092 can type is used for BC184 The X11 can type is used for BCX38B Transistors are given code names that are used to identify their safe operating characteristics. On the next page is an example of the BC184 transistor data sheet. It has much more information than we actually need for GCSE Systems and Control, but the important areas are high lighted. Milliamps or ma It is important to note that: 1 Amp = 1000mA So ½ Amp or 0.5 Amp = 500mA 0.1 Amp = 100mA Likewise 250mA = 0.25 Amps So 20mA (the maximum current allowed to flow through an LED) is 0.02A. The maximum current a transistor can handle between Collector and Emitter is rated in ma 40
How can I learn to remember the symbol? LOOK, COVER, REMEMBER and WRITE Use this method to practice drawing and labelling the transistor symbol. Base Collector Emitter hfe or current gain. A transistor also amplifies current, and is known as hfe. To calculate the amplification of a transistor the formula is: hfe = Ic Ib Ic is bigger than Ib Ib is always very small So if Ic = 100mA and Ib = 1mA hfe = Ic = 100 therefore the hfe is 100 Ib 1 41
Activity: Connect up the Nightlight Circuit Shown below on a breadboard. Input LDR to sense dark Process BC184 Output LED When the LDR is covered the LED will light. The 100K variable resistor can adjust the threshold of the circuit, so it will operate at different light levels. +6 100K 1K 330Ω Limiting Resistor OUTPUT Power Supply INPUT CIRCUIT PROCESS In order to make this circuit more responsive change the BC184 transistor for a BCX38B and write down what you see happening. 42
Your bread board layout might look like this: REMEMBER THE LED will only light when you cover the LDR 43
Darlington Pairs A BCX38B is a Darlington pair transistor. It contains 2 transistors in one. This means it can switch on and off very quickly. This can then provide very clear switching from one state to another. It would be useful for streetlights to switch them on and off, not gradually brighten. 44
SAQ Label this transistor with Collector Base and Emitter What Voltage will switch on a Transistor at its base? Use a ruler and draw a potential Divider circuit with Rtop = 100K and Rbot = 200K. Connect to a 9V battery. Use a calculator and calculate Voltage VRb 45
The Thyristor A three-legged component very similar to the Transistor, except it can latch on. This means when you apply a voltage to its gate leg it will conduct between Anode and Cathode permanently until the current is interrupted. It is made from four layers of semiconductor material as opposed to three with the transistor, however a small current at the gate will allow a large current to flow between the Anode and Cathode. Anode LARGE current P Type N Type small current gate P Type N Type Cathode The symbol for a Thyristor is : Anode gate Cathode 46
Crocodile clips Design a circuit to set off an alarm when some one presses a pressure mat, the circuit should latch on: My Input will be: My Process will be: My Output will be: My system block diagram looks like this: Paste your Crocodile clips model here: 47
PIC Chips We will cover PIC chips in more detail in Computer Control, however they are PROGRAMMABLE INTERFACE CONTROLLERS, or Peripheral Interface Controllers. They are self-contained micro controllers that can be programmed and reprogrammed to perform a number of different operations, or sequences. However they still need to be connected into an electronic circuit. In order to connect PIC chips to electronic circuitry you will need to know the PIN Diagram of the PIC chip you are using. PIC chips come in variety of sizes, described by the number of legs they have. In school we use 8 pin or 18 pin PIC chips. We use the PICAXE system of PIC chips. +6V Serial in Pin 4 Pin 3 1 3 4 2 PICAXE08 8 6 5 7 0V Pin 0 Pin 1 Pin 2 Each leg has a specific function but in essence Pin 0 to Pin 4 can be used as either inputs or outputs to a system. With the exception Pin 3 is always an input and Pin 0 is always an output. For more information on the PICAXE system see the Chapter on Computer Control or refer to www.picaxe.co.uk A circuit is drawn below illustrating how a PICAXE chip can be connected. 1 2 3 4 PICAXE08 8 7 6 5 Activity Study this circuit and suggest values for the resistors. Label the 6V battery, push button switch and LEDs. Label the legs from 1 to 8 from the Pin Diagram above. This circuit will do nothing until the PIC Chip has been programmed. 48
PICAXE Pin Diagrams 49
OP AMP The Operational Amplifier is a very high gain amplifier that can be used in a variety of modes. It comes in a similar package to the 8-pinned PIC chip, but is not programmable. PIN diagram of 741. 1 Offset Null 2 Inverting Input 3 Non inverting Input 4 0V (-V) 5 Offset Null 6 Output 7 +V 8 Not connected OP AMP symbol 1 2 3 4 8 7 6 5 +V LEG 7 Inverting input - V out Non - Inverting input + -V With any amplifier the gain of the amplifier (A v ) is given using the formula: A v = Ouput Voltage = V out Input Voltage = V in For example if Vin = 0.5 Volts and Vout = 7 Volts A v = Vout Vin Av = 7 0.5 Av = 14 50
Inverting Amp Rf Input Voltage Rin 0V - + +V -V Vout The gain in this case is given by the ratio of the feedback resistor (Rf) and the input resistor (Ri). Av = Rf Rin The output will always be inverted in respect to the input. If the input is negative the output will be postive and vice versa. Non Inverting amplifier +V Input Voltage - + -V Vout Rin Rf 0V The gain is set by the Rf and Rin. Av can be calculated using the formula: Av = 1+ Rf Rin If the input goes positive the output will go positive and the same of the negative input. The output will be an amplified version of the input 51
The Comparator or really sensitive switch. +9V -t - 10kΩ Ref + 4k7Ω 1kΩ In a practical comparator circuit the non-inverting input is set at a reference voltage, and the inverting input is connected to a potential divider circuit. In this way the OP AMP compares one voltage with another. Because there is no feedback resistor the circuit works at its maximum amplification (approximately 10,000) and amplifies the difference between the reference and the input voltage. So the output will swing between one supply voltage and another, in this case 0V and 9V. In practice it will never reach 9V and so a maximum of 7V can be measured. Most IC s can only sink or source 10mA, and so you will see a transistor used as a transducer driver to switch on the buzzer in this case. Sinking current means for an output to receive current, and source means to output current to a device. - + Sourcing current - + Sinking current 52
555 Timer The 555 integrated circuit is used as the main component in one of two multivibrator circuits. The monostable multivibrator used to switch something on or off for a certain length of time. The astable multivibrator, can be used as a pulse generator to provide a series of clock pulses for a counter or to turn something on and off continuously. The 555 is an 8 pinned DIL (Dual In Line) Integrated Circuit (IC) which consists of a silicon chip inside a plastic package. The circuit inside the 555 has two comparators, a bistable, resistors and transistors. The output at pin 5 can sink or source up to 300mA. 555 Symbol +V Threshold 6 8 Discharge 7 3 Output Trigger 2 1 0V Monostable mode The output of the 555 will be at 0V until the circuit is triggered. The trigger can come from almost any input transducer that sends a signal to the trigger pin. When the circuit is triggered the output will stay switched on for a set amount of time. +6 to +12 Volts 10kΩ R1 6 8 4 7 3 C1 2 1 53
Calculating the time delay for a Monostable circuit. T = 1.1 C 1 R 1 Where C is the value of capacitance and R is resistance. Let C 1 = 10µF and R 1 =100K T = 1.1 x (0.000 01 x 100 000) T = 1.1 x 1 T = 1.1s Astable circuit The number of pulses in one second (the frequency) is set by the values of C1, R1 and R2. +6 to +12 Volts R1 7 8 4 R2 6 3 C1 2 1 The output frequency can be calculated using this formula: F = 1.44 (R1 +2R2)C1 R1= 10 K R2 = 100K C1 = 1µF F = 1.44 (10 000 + 200 000) x 0.000 001 F = 1.44 0.21 F = 6.9 Hz (Hertz) (pulses per second) 54
Pin Diagram of the 555 Timer IC Ground 1 8 Vcc Trigger 2 7 discharge Output 3 6 threshold Reset 4 5 control 55
CROCODILE CLIPS ACTIVITY Using Crocodile clips modelling software draw out the MONOSTABLE circuit and plot a graph of the output. Your circuit should look like this: Pin 5 not connected By placing probes at the output and on the capacitor you can plot the charging rate of the capacitor and the length of time the output can stay on for. Probes Charging trace Output trace 56
Astable activity Now draw the Astable Circuit and print out your circuit and trace. Paste your work on this page. Make R1 = 10 K, R2 = 100K and C1 = 1µF Circuit Tracing 57
LOGIC GATES DIGITAL electronic signals are manipulated inside PIC chips and other microprocessor by a system of LOGIC GATES. Review what is a digital signal? However this is not the only place where digital signals are manipulated. Discrete components can be bought from Rapid Electronics with logic gates built inside and these circuits can be designed to perform a particular control function. The NOT gate The NOT gate inverts any digital signal entering at A and outputs the inversion at B A B Truth table for the NOT gate A B 1 0 0 1 LOGICALLY speaking the gate INVERTS the signal at A. 58
The AND Gate The AND gate will only give a 1 output when A AND B are both a digital 1. A B C Truth Table for the AND gate A B C 0 0 0 1 0 0 0 1 0 1 1 1 The NAND gate (NOT AND) This gate INVERTS the output of the AND gate and therefore will give a digital 0 output when A AND B are both digital 1. A B C Complete the Truth Table for the NAND gate A B C 0 0 1 0 0 1 1 1 59
The OR gate A B C This gate produces a digital 1 ouput when A B are a digital 1. Complete the Truth Table. A B C 0 0 0 1 0 0 1 1 1 1 1 1 The NOR gate The NOR gate produces a digital 1 only when A B C Complete the Truth Table A B C 0 0 1 0 0 1 1 1 60
EXOR gate (EXclusive OR) A B C Truth Table Describe the action of the EXOR gate. A B C 0 0 0 1 0 1 0 1 1 1 1 0 The EXOR gate provides a digital 1 when LOGIC SYSTEMS complete the truth table A B D F C E A B C D E F 0 0 0 0 1 1 0 0 1 0 0 1 0 1 0 1 1 0 0 1 1 1 0 1 1 0 0 1 1 0 1 0 1 1 0 1 1 1 0 1 1 0 1 1 1 1 0 1 61
Logic gates come in packages in integrated circuits. Typical codes are the: 4000 A dual 3 input NOR gate 4001 - A quad 2 input NOR gate and here is the pin diagram of such a chip. 62
SAQ 1. What 5 process components are covered in this section? (5) 2. What material are process components made of? (2) 3. What voltage switches on a transistor? (2) 4. Draw the symbol of a transistor (5) 5. Draw the pin diagram of a PIC AXE 08. (6) 6. What is an Op Amp (3) 7. What are the two modes of a 555 timer? (3) 8. Draw a NOR gate and explain what it does. (4) 63