1 Electronics 4.4: Digital Processes Transistors and Digital Electronics Electronics 4.4: Digital Processes

2 Introducing Transistors This is the symbol for an NPN transistor. 3 Transistors are process devices.

3 Electronics 4.4: Digital Processes Transistor Terminals Transistors have three terminals: Collector Emitter Base

4 Electronics 4.4: Digital Processes Transistor as a Switch Transistors can be used as switches. 1 Transistors can either conductnot conduct conduct or not conduct current. 2 onoffie, transistors can either be on or off. 2 TransistorSwitch

5 Electronics 4.4: Digital Processes How Transistors Work Switching is controlled by the voltage between the Base and the Emitter. Collector Emitter Base When V BE < 0.7V the transistor switches off and no current flows between the Collector and the Emitter. When V BE ≥ 0.7V the transistor switches on and current flows between the Collector and the Emitter.

6 Electronics 4.4: Digital Processes Transistor Switching Example 15 When V BE is less than 0.7V the transistor is off and the lamp does not light. When V BE is greater than 0.7V the transistor is on and the lamp lights. X Variable Voltage Supply 12V

7 Electronics 4.4: Digital Processes Transistor Circuit #1: Temperature-Controlled Circuit This transistor circuit contains a Thermistor. Because of the thermistor, this circuit is dependent on temperature. The purpose of this circuit is to turn on the LED when the temperature reaches... Input= Voltage Divider Process= Transistor Output= LED 1)LED = Off. 2)Heat the Thermistor. 3)R Thermistor . 4)V Thermistor . 5)Voltage across 10k  resistor . 6)Transistor switches on. 7)LED = On.

8 Electronics 4.4: Digital Processes Transistor Circuit #2: Light-Controlled Circuit This transistor circuit contains a Light-Dependent Resistor. Because of the LDR, this circuit is dependent on light. The purpose of this circuit is to turn on the LED when the light reaches a certain intensity. Input= Voltage Divider Process= Transistor Output= LED 1)LED = Off. 2)Cover LDR. 3)R LDR . 4)V LDR . 5)Transistor switches on. 6)LED = On.

9 Electronics 4.4: Digital Processes Transistor Circuit #3: Time-Controlled Circuit This transistor circuit contains a Capacitor. Because of the capacitor, this circuit is dependent on the time taken to charge and discharge of the capacitor. The purpose of this circuit is to turn on the LED a short time after the switch is opened. Where would this circuit be found in a car? Input= Voltage Divider Process= Transistor Output= LED 5)Open Switch. 6)V C . 7)Transistor switches on after a short delay. 8)LED = On. 1)Switch closed. 2)V C = 0V. 3)Transistor switches off. 4)LED = Off.

10 Electronics 4.4: Digital Processes Summary of Transistor Switching Circuits You are expected to know the purpose of a transistor switching circuit: the last three pages should help. 4 In each of the three circuits the input device is: A Voltage Divider using a Thermistor LDR Capacitor In each of the three circuits the output device is:an LED

11 Electronics 4.4: Digital Processes Logic

12 Electronics 4.4: Digital Processes From Section 4.2 Output Devices, remember that digital signals have only two values, 7 “1” and “_”, or “High Voltage” and “___ _______”, or “On” and “___”, or “True” and “_____”. Off On High Voltage Low Voltage Revision: Digital Signals 1 0

13 Electronics 4.4: Digital Processes Introduction to Logic Many digital electronic processes are designed around “logic” circuits. The Inputs and Outputs in logic have only two values: 0 & 1; High & Low; On & Off; True and False. Logic is ideally suited to help design digital electronic circuits because of its binary nature. We will look at some fundamental logic circuits.

14 Electronics 4.4: Digital Processes Logic: Switches in Series The bulb will light only under certain conditions: what? S1S2 Complete the following: The bulb will turn on only when switches S1 ___ S2 are closed, for all other combinations the bulb is off. S1S2Lit

15 Electronics 4.4: Digital Processes Logic: Switches in Parallel S1 S2 The bulb will light under certain conditions: what? Complete the following: The bulb will turn on when switches S1 ___ S2 are closed, for all other combinations the bulb is off. S1S2Lit

16 Electronics 4.4: Digital Processes Logic: Opposites! S The bulb will light under certain conditions: what? Complete the following: The bulb will turn on when switch S is ____, and turn off when switch S is ______. SLit This circuit is for illustration only! If this was a real circuit, what would happen to the battery when switch S was closed?

17 Electronics 4.4: Digital Processes Truth Tables The tables on the previous pages are truth tables. Truth Tables list: All combinations of all possible inputs, Every Output for each combination of inputs. There are special circuits called logic gates which can be used in control situations. S1S2Lit S1S2Lit S1Lit 01 10

18 Electronics 4.4: Digital Processes Logic Gates: AND Two-Input AND Gate 5 AND Truth Table 8,16 ABQ The output of an AND gate is 1 only when all inputs are 1. Only when Input A AND Input B are 1, the output is 1. See page “Logic: Switches in Series”.

19 Electronics 4.4: Digital Processes Logic Gates: OR Two-Input OR Gate 5 OR Truth Table 8,16 ABQ The output of an OR gate is 1 when any input is 1. When Input A OR Input B is 1, the output is 1. See page “Logic: Switches in Parallel”.

20 Electronics 4.4: Digital Processes Logic Gates: NOT NOT Gate 5 NOT Truth Table 8,16 AQ The output of a NOT gate is the opposite of the input. When Input A is 0, the output is 1. When Input A is 1, the output is 0 Note that NOT gates have only one input. See page “Logic: Opposites!”.

21 Electronics 4.4: Digital Processes Summary of Logic Gates and Truth Tables Truth Tables list: 6 Every Outputevery combinationEvery Output for every combination of inputs. AND Gate ABQ OR Gate ABQ NOT Gate AQ Logic gates may have one or more inputs. 6

22 Electronics 4.4: Digital Processes Combinational Logic Circuits 9,17 Combinational Logic Circuits are simply circuits using a combination of AND, OR and NOT gates. You are expected to design Logic Circuits and Truth Tables of simple combinational logic circuits.

23 Electronics 4.4: Digital Processes Logic Circuit #1: Car ’ s Hot Engine When a car’s engine becomes too hot an LED should light but only when the ignition is switched on. LED Ignition Switch Temperature Sensor Truth Table IgnitionTemperatureOutput SwitchSensorLED OffColdOff OffHotOff OnColdOff OnHotOn Here, the truth table is simply that for an AND Gate. For the LED to light, the Ignition Switch must be on and the Temperature Sensor must be “ hot ”. 1 1

24 Electronics 4.4: Digital Processes Logic Circuit #2: Central Heating Pump Derive a logic circuit that will turn on a Central Heating System ’ s pump when the house is cold and the Central Heating System is turned on. This time let ’ s find the truth table first: House is Cold = 0 ; House is Hot = 1 CHS is Off = 0; CHS is On = 1 Truth Table HouseCHSPump ColdOffOff ColdOnOn HotOffOff HotOnOff HouseCHSPump Central Heating Pump 1 10 Temperature Sensor

25 Electronics 4.4: Digital Processes Heater Logic Circuit #3: Greenhouse Heater Derive a logic circuit that will turn on a heater in a greenhouse only when it gets cold at night. Truth Table: Greenhouse Cold = 0 ; Hot = 1 Dark = 0; Light = 1 Truth Table GreenDay/ Heater houseNight ColdNightOn ColdDayOff HotNightOff HotDayOff GreenD/N Heater Light Sensor Temperature Sensor

26 Electronics 4.4: Digital Processes Summary of Combinational Logic Circuits Combinational Logic Circuits are simply combinations of AND, OR and NOT gates. Constructing Logic Circuits 1)Make a Truth Table. 2)Get the logic circuit from the Truth Table. Tip: If the circuit has only one “ high ” output then the circuit will probably use an AND Gate. Tip: If the circuit has more than one “ high ” output then the circuit will probably use an OR Gate. Tip: Note how useful NOT gates are!

27 Electronics 4.4: Digital Processes Clocks Clocks are normally square waves. Clocks are regular waves of pulses, just like the ticking of a conventional clock: Digital circuits can be used to produce a series of clock pulses. 10 The circuits which produce clock pulses are sometimes called oscillators because they constantly oscillate between “ on ” and “ off ”.

28 Electronics 4.4: Digital Processes A Simple Oscillator Circuit Supply Voltage V S = V 1 +V 2 When the NOT-Gate outputs a 0, V 2 =0V and V 1 =5V: the LED lights. When the NOT-Gate outputs a 1, V 2 =5V and V 1 =0V: the LED does not light. Oscillator Circuits change between two values in a regular cyclical pattern: a clock output.

29 Electronics 4.4: Digital Processes How an Oscillator Circuit Works 18 1)The Invertor ’ s Input is 1, so its Output = 0: the Capacitor starts to discharge through the Resistor. Capacitor C charges and discharges through Resistor R. Start: ASSUME THE CAPACITOR IS FULLY CHARGED. 2)As the Capacitor discharges the Invertor ’ s Input eventually falls to 0, so its Output becomes 1: the Capacitor starts to charge through the Resistor. 3)As the Capacitor charges the Invertor ’ s Input eventually rises to 1, so its output becomes 0: the Capacitor discharges again. 4)This sequence of charging and discharging continues ad infinitum to produce a series of clock pulses.

30 Electronics 4.4: Digital Processes How an Oscillator Circuit Works (Alternative) CapacitorNOTNOTV2V1LED InputOutput Charged100V5VOn Discharged015V0VOff Charged100V5VOn Discharged015V0VOff

31 Electronics 4.4: Digital Processes How to Change a Clock ’ s Frequency 19 The frequency of clock pulses can be altered: High FrequencyLow Frequency If the value of the Capacitor is increased, charging and discharging takes longer so the clock frequency is decreased. If the value of the Resistor is increased, charging and discharging takes longer so the clock frequency is decreased. C  then f  R  then f 

32 Electronics 4.4: Digital Processes Counters Counters are electronic circuits which can count digital pulses from a clock. 12 Counters count the clock pulses in binary

33 Electronics 4.4: Digital Processes Counting in Decimal Binary Decimal etc etc Binary-to-Decimal ConvertorsCircuits called Binary-to-Decimal Convertors convert a counter ’ s binary output into decimal. 14

34 Electronics 4.4: Digital Processes Devices using Counters You will be expected to name a device which uses a counter. The most common device to use a counting circuit is an electronic clock or watch. 11 Electronic timing devices work with great accuracy. An electronic watch ’ s “ clock circuit ” generates regular pulses and a Counter simply counts these. The watch ’ s microprocessor is programmed to know how many clock pulses correspond to a second (and minute, hour etc) and will update the 7-segment display accordingly: thus displaying the time!