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.

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

Automatic Night Light

Experiment

An automatic night light is constructed as shown.

VS

0 V

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

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.

Temperature Control

Experiment

An automatic temperature control circuit is constructed as shown.

VS

0 V

Vtherm

VVR

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

Yellow Book

Switching Circuits – Page 50

Q36, Q37, Q38, Q39

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.

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

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

Yellow Book

Switching Circuits – Page 51

Q40 and Q41

AND Gate

The symbol for an AND gate is:

A

BZ

A and B are inputs to the AND gate.

Z is the output.

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 )

OR Gate

The symbol for an OR gate is:

A

BZ

A and B are inputs to the OR gate.

Z is the output.

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 )

NOT Gate

The symbol for a NOT gate is:

Z

A is the input to the NOT gate.

Z is the output.

A

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.

Automatic Night Light

Experiment

LDR

Results

Light Level LDR Bulb

bright 1 0

dark 0 1

Night Light With Master Switch

Experiment

LDR

X

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

Night Light With Test Switch

Experiment

LDR

A X

B

Z

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

Combining Logic Gates

Example 1

Complete a truth table for the following combination of logic gates.

AB

C

X

Z

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.

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

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.

Yellow Book

Logic Gates – Page 52

Q43, Q44, Q47, Q48, Q50, Q52

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 )

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

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

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

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.

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

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.

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.

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

Yellow Book

Clock Pulse Generators – Page 55

Q59, Q60, Q61, Q62