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Transcript
Electronics 4.4: Digital Processes Contents Page Title 1) 2) 3) Contents Page Header Page Learning Outcomes 4) 5) 6) 7) 8) 9) 10) 11) 12) Introducing Transistors Transistor Terminals Transistor as a Switch How Transistors Work Transistor Switching Example Transistor Circuit #1: Temperature-Controlled Circuit Transistor Circuit #2: Light-Controlled Circuit Transistor Circuit #3: Time-Controlled Circuit Summary of Transistor Switching Circuits 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23) Logic Revision: Digital Signals Introduction to Logic Logic: Switches in Series Logic: Switches in Parallel Logic: Opposites! Truth Tables Logic Gates: AND Logic Gates: OR Logic Gates: NOT Summary of Logic Gates and Truth Tables 24) 25) 26) 27) 28) Combinational Logic Circuits Logic Circuit #1: Car’s Hot Engine Logic Circuit #2: Central Heating Pump Logic Circuit #3: Greenhouse Heater Summary of Combinational Logic Circuits Page Title 29) 30) 31) 32) 33) Clocks A Simple Oscillator Circuit How an Oscillator Circuit Works How an Oscillator Circuit Works (Alternative) How to Change a Clock’s Frequency 34) 35) 36) Counters Counting in Decimal Devices Using Counters 1 Electronics 4.4: Digital Processes Electronics 4.4: Digital Processes Electronics Section 4.4 Digital Processes 2 Electronics 4.4: Digital Processes 4.4 Digital Processes: Learning Outcomes Transistor as a Switch 1) (G) State that a transistor can be used as a switch. 2) (G) State that a transistor may be conducting or nonconducting, ie on or off. Simple Switching Systems 3) (G) Draw and identify the circuit symbol for an NPN transistor. 4) (G) Identify from a circuit diagram the purpose of a simple transistor switching circuit. 15) (C) Explain the operation of a simple transistor switching circuit. Digital Logic Gates 5) (G) Draw and identify the symbols for two-input AND, OR and NOT gates. 6) (G) State that logic gates may have one or more inputs and that a truth table shows the output for all possible input combinations. 7) (G) State that high voltage = logic 1, low voltage = logic 0. 8) (G) Draw the truth tables for AND OR and NOT gates. 16) (C) Identify the following gates from truth tables: AND, OR, NOT. Combinational Logic Circuits 9) (G) Explain how to use combinations of digital logic gates for control in simple situations. 17) (C) Complete a truth table for a simple combinational logic circuit. Clock Signals 10) (G) State that a digital circuit can produce a series of clock pulses. 18) (C) Explain how a simple oscillator built from a Resistor, Capacitor and Inverter operates. 19) (C) Describe how to change the frequency of a clock. Counters 11) (G) Give an example of a device containing a counter circuit. 12) (G) State that there are circuits which can count digital pulses. 13) (G) State that the output of the counter circuit is in binary. 14) (G) State that the output of a binary counter can be converted to decimal. 3 Electronics 4.4: Digital Processes Introducing Transistors • Transistors are process devices. • This is the symbol for an NPN transistor.3 4 Electronics 4.4: Digital Processes Transistor Terminals • Transistors have three terminals: Collector Base Emitter 5 Electronics 4.4: Digital Processes Transistor as a Switch • Transistors can be used as switches.1 Transistor Switch • Transistors can either conduct or not conduct current.2 • ie, transistors can either be on or off.2 6 Electronics 4.4: Digital Processes How Transistors Work Collector • Switching is controlled by the voltage between the Base and the Emitter. Base Emitter • When VBE < 0.7V the transistor switches off and no current flows between the Collector and the Emitter. • When VBE ≥ 0.7V the transistor switches on and current flows between the Collector and the Emitter. 7 Electronics 4.4: Digital Processes Transistor Switching Example15 X 12V Variable Voltage Supply • When VBE is less than 0.7V the transistor is off and the lamp does not light. • When VBE is greater than 0.7V the transistor is on and the lamp lights. 8 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) 2) 3) 4) 5) 6) 7) LED = Off. Heat the Thermistor. RThermistor . VThermistor . Voltage across 10k resistor . Transistor switches on. LED = On. 9 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) 2) 3) 4) 5) 6) LED = Off. Cover LDR. RLDR . VLDR . Transistor switches on. LED = On. 10 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. Input = Voltage Divider Process = Transistor Output = LED 1) 2) 3) 4) Switch closed. VC = 0V. Transistor switches off. LED = Off. • Where would this circuit be found in a car? 5) Open Switch. 6) VC . 7) Transistor switches on after a short delay. 8) LED = On. 11 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 12 Electronics 4.4: Digital Processes Logic 13 Electronics 4.4: Digital Processes Revision: Digital Signals • 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 “_____”. On 1 Off 0 High Voltage Low Voltage 14 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. 15 Electronics 4.4: Digital Processes Logic: Switches in Series S1 S2 S1 S2 Lit 0 0 1 1 0 1 0 1 0 0 0 1 • The bulb will light only under certain conditions: what? Complete the following: • The bulb will turn on only when switches S1 ___ S2 are closed, for all other combinations the bulb is off. 16 Electronics 4.4: Digital Processes Logic: Switches in Parallel S1 S1 S2 Lit 0 0 1 1 0 1 0 1 0 1 1 1 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. 17 Electronics 4.4: Digital Processes Logic: Opposites! S S 0 1 Lit 1 0 • 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 ______. • This circuit is for illustration only! • If this was a real circuit, what would happen to the battery when switch S was closed? 18 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. S1 S2 Lit S1 S2 Lit S1 Lit 0 0 1 1 0 1 0 1 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 0 1 1 0 19 Electronics 4.4: Digital Processes Logic Gates: AND AND Truth Table8,16 Two-Input AND Gate5 A B Q 0 0 1 1 0 1 0 1 0 0 0 1 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”. 20 Electronics 4.4: Digital Processes Logic Gates: OR OR Truth Table8,16 Two-Input OR Gate5 A B Q 0 0 1 1 0 1 0 1 0 1 1 1 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”. 21 Electronics 4.4: Digital Processes Logic Gates: NOT NOT Truth Table8,16 NOT Gate5 A Q 0 1 1 0 Note that NOT gates have only one input. 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 • See page “Logic: Opposites!”. 22 Electronics 4.4: Digital Processes Summary of Logic Gates and Truth Tables • Logic gates may have one or more inputs.6 AND Gate A B Q 0 0 1 1 0 1 0 1 0 0 0 1 OR Gate A B Q 0 0 1 1 0 1 0 1 0 1 1 1 NOT Gate A Q 0 1 1 0 Truth Tables list:6 • Every Output for every combination of inputs. 23 Electronics 4.4: Digital Processes Combinational Logic Circuits9,17 • 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. 24 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. Ignition Switch 1 LED Temperature 1 Sensor Truth Table Ignition Temperature Switch Sensor Off Off On On Cold Hot Cold Hot Output LED Off Off Off On • 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”. 25 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 Central Heating 1 0 Temperature Sensor 1 Pump Truth Table House CHS Pump Cold Cold Hot Hot Off On Off On Off On Off Off House CHS Pump 0 0 1 1 0 1 0 1 0 1 0 0 26 Electronics 4.4: Digital Processes 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 Light Sensor Temperature Sensor 0 1 0 1 Truth Table Green Day/ Heater house Night Heater Cold Cold Hot Hot Night Day Night Day On Off Off Off Green D/N Heater 0 0 1 1 0 1 0 1 1 0 0 0 27 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! 28 Electronics 4.4: Digital Processes Clocks • 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 • Clocks are normally square waves. • The circuits which produce clock pulses are sometimes called oscillators because they constantly oscillate between “on” and “off”. 29 Electronics 4.4: Digital Processes A Simple Oscillator Circuit • Oscillator Circuits change between two values in a regular cyclical pattern: a clock output. • Supply Voltage VS = V1 +V2 • When the NOT-Gate outputs a 0, V2=0V and V1=5V: the LED lights. • When the NOT-Gate outputs a 1, V2=5V and V1=0V: the LED does not light. 30 Electronics 4.4: Digital Processes How an Oscillator Circuit Works18 • Capacitor C charges and discharges through Resistor R. •Start: ASSUME THE CAPACITOR IS FULLY CHARGED. 1) The Invertor’s Input is 1, so its Output = 0: the Capacitor starts to discharge through the Resistor. 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. 31 Electronics 4.4: Digital Processes How an Oscillator Circuit Works (Alternative) Capacitor NOT Input NOT Output V2 V1 LED Charged 1 0 0V 5V On Discharged 0 1 5V 0V Off Charged 1 0 0V 5V On Discharged 0 1 5V 0V Off 32 Electronics 4.4: Digital Processes How to Change a Clock’s Frequency19 • The frequency of clock pulses can be altered: High Frequency Low Frequency • If the value of the Capacitor is increased, charging and discharging takes longer so the clock frequency is decreased. C then f • If the value of the Resistor is increased, charging and discharging takes longer so R then f the clock frequency is decreased. 33 Electronics 4.4: Digital Processes Counters • Counters are electronic circuits which can count digital pulses from a clock.12 1 2 3 4 • Counters count the clock pulses in binary.13 34 Electronics 4.4: Digital Processes Counting in Decimal • Circuits called Binary-to-Decimal Convertors convert a counter’s binary output into decimal.14 Binary Decimal 0000 0001 0010 etc etc 0111 1000 1001 35 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! 36 Electronics 4.4: Digital Processes Future Improvements • Clip Art • Imagination • Summarise 37