the transistor a transistor is an automatic switch. it can only be on or off. base emitter collector...
TRANSCRIPT
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The Transistor
A transistor is an automatic switch.
It can only be on or off.
base
emitter
collector
0.7 V
When the transistor is on, current flows from the emitter to the collector.
For this to happen, there has to be 0.7 V across the base of the transistor.
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If the voltage across the base is less than 0.7 V, the transistor is switched off and no current flows.
V0.7 Vbase OFF transistor
V0.7 Vbase ON transistor
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Automatic Night Light
Experiment
An automatic night light is constructed as shown.
VS
0 V
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How It Works
In bright light:
• resistance of the LDR is low (LURD)
• if resistance of LDR is low, this means voltage across LDR is also low.
• base voltage is low ( < 0.7 V).
• transistor is OFF
• LED does not light
In dim light:
• resistance of LDR increases
• voltage across LDR increases
• base voltage increases ( > 0.7 V)
• transistor switches ON causing LED to light.
VS
0 V
VVR
VLDR
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Devices
Input:
Process:
Output:
voltage divider
transistor
LED
This circuit switches on a light when it is dark and switches it off when it is light.
Putting the LDR at the top (reversing the components) makes the circuit do the opposite.
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Temperature Control
Experiment
An automatic temperature control circuit is constructed as shown.
VS
0 V
Vtherm
VVR
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How It Works
As the temperature increases:
• resistance across thermistor falls (TURD)
• this means voltage across thermistor falls
• voltage across variable resistor increases
• base voltage increases ( > 0.7 V)
• transistor switches ON
• LED lights.
This circuit switches on a light when the temperature increases.
Putting the thermistor at the bottom (reversing the components) makes the circuit do the opposite.
VS
0 V
Vtherm
VVR
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Yellow Book
Switching Circuits – Page 50
Q36, Q37, Q38, Q39
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Time Delay Circuits
A time delay circuit is constructed as shown.
VS
0 V
switch
R
C
The CAPACITOR is the input device responsible for
the TIME DELAY.
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How It Works
Switch Open
• the capacitor begins to charge up
• voltage across capacitor increases
• base voltage takes several seconds to reach 0.7 V
• as it does so, transistor switches on
• LED lights.
Switch Closed
• capacitor discharges
VS
0 V
switch
R
C
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Putting the capacitor at the top (reversing the components) makes the circuit do the opposite.
The light would switch off after a time delay.
Size of Time Delay
Change to Circuit Effect
increase size of R increases time delay
decrease size of R decreases time delay
increase size of C increases time delay
decrease size of C decreases time delay
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Yellow Book
Switching Circuits – Page 51
Q40 and Q41
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AND Gate
The symbol for an AND gate is:
A
BZ
A and B are inputs to the AND gate.
Z is the output.
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Experiment
An AND gate is connected to a light source as shown.
AZ
B
Results
The results are recorded in a truth table.
A B Z
0 0 0
1 0 0
0 1 0
1 1 1
The output is only a high voltage ( 1 ) when
BOTH input A AND input B
are connected to a high voltage ( 1 )
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OR Gate
The symbol for an OR gate is:
A
BZ
A and B are inputs to the OR gate.
Z is the output.
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Experiment
An OR gate is connected to a light source as shown.
AZ
B
Results
The results are recorded in a truth table.
A B Z
0 0 0
1 0 1
0 1 1
1 1 1
The output is a high voltage ( 1 ) when
input A OR input B
are connected to a high voltage ( 1 )
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NOT Gate
The symbol for a NOT gate is:
Z
A is the input to the NOT gate.
Z is the output.
A
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Experiment
A NOT gate is connected to a light source as shown.
A
Results
The results are recorded in a truth table.
A Z
0 1
1 0
The output is a high voltage ( 1 ) when
input A is NOT
connected to a high voltage ( 1 )
Z
The NOT gate is also known as an INVERTOR, as it inverts the input. Changes 0 to 1 or vice versa.
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Automatic Night Light
Experiment
LDR
Results
Light Level LDR Bulb
bright 1 0
dark 0 1
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Night Light With Master Switch
Experiment
LDR
X
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Results
The results are recorded in a truth table.
LDR Switch X Bulb
0 0 1
0 1 1
1 0 0
1 1 0
0
1
0
0
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Night Light With Test Switch
Experiment
LDR
A X
B
Z
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Results
The results are recorded in a truth table.
A B X Z
0 0 1 1
0 1 1 1
1 0 0 0
1 1 0 1
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Combining Logic Gates
Example 1
Complete a truth table for the following combination of logic gates.
AB
C
X
Z
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A B C X Z
0 0 0 0 0
1 0 0 0 0
0 1 0 0 0
0 0 1 0 1
0 1 1 0 1
1 0 1 0 1
1 1 0 1 1
1 1 1 1 1
Gate 1 (AND)
A and B are the inputs.
X is the output.
Gate 2 (OR)
X and C are the inputs.
Z is the output.
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Example 2
The following circuit is a combination of logic gates.
(a) State the name of components 1, 2 and 3.
(b) Label your circuit diagram with inputs and outputs.
(c) Complete a truth table for the circuit shown.
1 23
1 = NOT gate 2 = OR gate 3 = AND gate
A
B
CX
YZ
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A B C X Y Z
0 0 0 1 1
1 0 0 1 1
0 1 0 0 0
0 0 1 1 1
0 1 1 0 0
1 0 1 1 1
1 1 0 0 1
1 1 1 0 1
0
0
0
1
0
1
0
1
Gate 1 (NOT)
B is the input.
X is the output.
Gate 2 (OR)
A and X are the inputs.
Y is the output.
Gate 3 (AND)
Y and C are the inputs.
Z is the output.
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Yellow Book
Logic Gates – Page 52
Q43, Q44, Q47, Q48, Q50, Q52
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Designing Circuits
Example 1
Draw a circuit and truth table that will switch on a warning LED when a car engine gets too hot.
It should only operate when the ignition switch is closed.
Temperature Sensor
( Hot - 1 )
Light sensor
(Light - 1 )
Switch
( Closed - 1 )
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A
B
Z
Temperature Sensor
LED
A B Z
0 0 0
0 1 0
1 0 0
1 1 1
open & cold
open & hot
closed & cold
closed & hot
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Example 2
Draw a circuit and truth table that will switch on a central heating system when it is cold, or switched on manually.
A
X
ZB
A B X Z
0 0 1 1
0 1 0 0
1 0 1 1
1 1 0 1
Temperature Sensor
HEATING
open & cold
open & hot
closed & cold
closed & hot
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Q1. Design a circuit that will switch a motor on to open greenhouse windows when it is daylight and gets too hot.
Give the corresponding truth table for your circuit.
A
B
Z
Temperature Sensor
MOTOR
Light Sensor
A B Z
0 0 0
0 1 0
1 0 0
1 1 1
dark & cold
dark & hot
light & cold
light & hot
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Clock Pulse Generator
The clock pulse generator produces a series of pulses that can be used in timing devices.
C
R
output1
0
X
Y
X is the input to the NOT gate.
Y is the output.
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Circuit Operation
• initially capacitor is uncharged
• capacitor charges
• capacitor discharges
• process repeats over and over again.
X = 0 Y = 1
X = 1 Y = 0
X = 0 Y = 1
C
R
outputX
Y
Uses
Such counting circuits are essential in devices such as:
• digital watches
• computers
• timing of traffic lights
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Frequency of Pulses
The frequency of clock pulses depends on the size of resistor and capacitor.
Increasing R or C
It now takes longer for the capacitor to charge
and discharge.
Decreasing R or C
It now takes less time for the capacitor to charge
and discharge.
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Digital Clock
A digital clock uses a clock pulse generator that has a period of 1 second.
Clock Pulse Generator
1 sec
The output from the clock pulse generator is binary.
Using a decoder and a 7-segment display, we can convert to decimal form.
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Reset The Counter
The binary counter has a reset terminal which resets the counter to zero when the input to the reset terminal is high (1).
If the counter is connected to a 7-segment display, the counter would reset after the number 9.
This is achieved using an AND gate.
5 pulses = 5 sec
1248
Binary Counter
R
Decoder
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Yellow Book
Clock Pulse Generators – Page 55
Q59, Q60, Q61, Q62