KS4 Physics

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Electronic Control

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2 of 39 Contents

Electronic Control Controlling current

Electronic control systems Logic gates Bistable latch circuits Summary activities

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3 of 39 Switches

A switch is a simple device which makes it possible to control the current flowing in a circuit. When the switch is open, it creates a gap in the circuit which stops current flowing.

When the switch is closed, it closes the gap and allows current to flow.

The switch in this circuit controls the lamp.

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4 of 39 Relays

A relay is an electromagnetic switch which allows one circuit to control another circuit by turning it on and off.

A relay is used when it is necessary to operate a high power circuit with a small switch and thin wire.

The starter motor in a car uses a current of several hundred amps. A switch to control that much current would have to be very large, so a relay is used instead.

In this way, a small switch can be used to start the car.

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5 of 39 Relays

A relay uses an electromagnet to operate a switch.

coil steel switch M

As electricity flows in the input circuit, the coil becomes an electromagnet.

The coil attracts the steel switch in the output circuit and completes the second circuit.

Electricity flows in the output circuit and so the motor works.

input output Circuit symbol for a relay © Boardworks Ltd 2005

Parts of a relay spring coil of wire input connections 6 of 39 soft iron armature switch contacts output connections © Boardworks Ltd 2005

7 of 39 Combining switches

More than one switch can be added to a circuit to give more control over the output. If there are alternative routes that the current can take, a combination of switches is used to control these.

+ -

Switches at the top and bottom of a flight of stairs, which control the same light, are an example of this type of circuit.

If the light is on, operating either of the switches will turn it off.

If the light is off, operating either of the switches will turn it on.

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Combining switches 8 of 39

Car doors are fitted with switches that operate the interior light.

In this circuit, opening either door or both doors will turn on the light. The light will only go out when both doors are shut.

door B switch door A door B door A switch light

The table shows the state of the light for all the combinations of the doors being open or closed.

0 0 0 1 1 0 off on on

A closed door (open switch) is denoted by a 0 and an open door (closed switch) by a 1.

1 1 on © Boardworks Ltd 2005

9 of 39 Contents

Electronic control

Controlling current

Electronic control systems

Logic gates Bistable latch circuits Summary activities

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10 of 39 Parts of an electronic control system

A control system has three parts:

input sensor

This detects changes that the system responds to.

processor

This processes information from the input sensor and operates the output device.

output device

This device is controlled by the processor.

How many different input sensors and the output devices can you think of?

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11 of 39 Input sensors – switches

Switches that are activated by pressure, tilt, magnetic fields or moisture may be used as input sensors. A

reed switch

consists of two springy metal strips in a sealed glass tube. A magnetic field near the switch makes the strips come together.

A

mercury switch

consists of some mercury and electrical contacts in a sealed glass tube. When the switch is tilted in a certain direction, the liquid mercury touches the contacts, and completes the circuit.

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12 of 39 Input sensors – resistors

The resistance of a

light dependent resistor

(

LDR

) depends on the intensity of the light shining on it.

This makes an LDR useful in circuits which are controlled by light intensity.

The resistance of a

thermistor

depends on its temperature.

This makes it useful in circuits which monitor and control temperatures.

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13 of 39 Potential dividers

In a potential divider, the input sensor is connected in series with a resistor.

V

in

This type of device turns a change in resistance in the input device into a change in voltage.

This change in voltage may then be used to input information into the processor.

0V R

1

R

2

V

out

0V

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14 of 39 Potential dividers

A processor treats all voltages below its threshold voltage as LOW signals and above its threshold voltage as HIGH signals.

V

in

By using a variable resistor, the level at which the input sensor causes V out to reach the threshold voltage can be adjusted. This is useful in a thermostat

0V

where the temperature that triggers a heater can be adjusted.

V

out

0V

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15 of 39 Output devices – light

A

filament lamp

is a common output device which produces light. Filament lamps are capable of producing large amounts of light but are not very efficient.

A

light-emitting diode

(

LED

) is a type of diode which gives out light when current is passed through it. LEDs require less current and do not produce as much heat as filament lamps. They are often used as indicators and in displays.

Circuit symbol for an

LED © Boardworks Ltd 2005

Output devices – movement 16 of 39

A

motor

is a device that is used to produce movement.

Circuit symbol for a motor M

Motors, especially those that do a lot of work, require high currents and voltages to work. If a motor is being controlled by a processor, the processor can be damaged by the high current used to power the motor.

A relay is used to prevent high current damaging the processor.

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17 of 39 Output devices – sound and heat

Electric buzzers or loudspeakers are output devices that can be used to produce sound.

Circuit symbol for a b

uzzer

Electric heaters are output devices that can be used to produce heat.

Circuit symbol for a

heater © Boardworks Ltd 2005

18 of 39 Components of control systems © Boardworks Ltd 2005

19 of 39 Contents

Electronic control

Controlling current Electronic control systems

Logic gates

Bistable latch circuits Summary activities

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20 of 39 What are logic gates?

Logic gates are electronic switches that process information. They are called gates because they open and produce an output signal based on the correct combination of input signals.

This chip contains 4 NOT gates.

Logic gates are used in a wide range of devices including computers. A large number of logic gates can be incorporated in one electronic chip.

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21 of 39 What is an AND gate?

An AND gate has two inputs, A and B, and one output.

input A input B output

The output is only high when A

AND

B are high.

0 0 0 1 0 0 An AND logic circuit 1 1 0 1 Truth table for an AND gate 0 1 Circuit symbol for an AND gate A B © Boardworks Ltd 2005

22 of 39 What is an OR gate?

An OR gate has two inputs, A and B, and one output. The output is high when either A

OR

B is high.

An OR logic circuit input A input B output 0 0 0 0 1 1 1 1 0 1 Truth table for an OR gate 1 1 Circuit symbol for an OR gate A B © Boardworks Ltd 2005

23 of 39 What is a NOT gate?

A NOT gate has only one input and one output. The output is always the

opposite

of its input, so a NOT gate is called an

inverter

.

NOT gates are often used in combination with other logic gates.

input 0 1 output 1 0 Truth table for a NOT gate Circuit symbol for a NOT gate © Boardworks Ltd 2005

24 of 39 What is a NAND gate?

A NAND gate is a

NOT

gate and a

AND

gate combined.

input A input B output 0 0 1

A NAND gate has two inputs and one output.

0 1 1

The outputs are the opposite to those that would be obtained with an AND gate.

1 1 0 1 1 0 Truth table for a NAND gate Circuit symbol for a NAND gate A B © Boardworks Ltd 2005

25 of 39 What is a NOR gate?

A NOR gate is a

NOT

gate and a

OR

gate combined.

A NOR gate has two inputs and one output.

The outputs are the opposite to those that would be obtained with an AND gate.

input A input B output 0 0 1 0 1 0 1 1 0 1 Truth table for a NOR gate 0 0 Circuit symbol for a NOR gate A B © Boardworks Ltd 2005

26 of 39 The alternative guide to logic gates!

What are the names of the different logic gates in this scene?

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27 of 39 Combining logic gates

Logic gates are normally used in combination.

Consider a system in which a heater is turned when it is cold (

A

) and light (

B

). The system must also have a master switch (

C

) which allows the system to be switched on.

A B A 0 0 0 0 1 1 1 1 B 0 0 1 1 0 0 1 1 D C 0 1 0 1 0 1 0 1 C D 0 0 0 0 0 0 1 1 E 0 1 0 1 0 1 1 1 E © Boardworks Ltd 2005

28 of 39 Combining logic gates problem

Consider a system in which an alarm sounds if it is cold or raining. The temperature sensor gives a low signal if it is cold and the rain sensor gives a high signal if it is raining. There should also be a switch which turns on the alarm. How could logic gates be connected to control this alarm?

There are two possible circuits… temperature rain temperature switch switch rain © Boardworks Ltd 2005

29 of 39 Contents

Electronic control

Controlling current Electronic control systems Logic gates

Bistable latch circuits

Summary activities

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30 of 39 What is a bistable latch circuit?

A bistable latch is a simple memory circuit. It consists of two NOR gates that are connected together. S = 1 1 0 The circuit has two inputs S (set) and R (re-set). R = 0 There are two stable states which are set by a high input on S or R. S = 0 0 If the high input stops, the bistable stays (is latched) in that state.

R = 1 1 0 output = 1 1 output = 0

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A bistable latch circuit – two NOR gates

A

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B R = 0 Imagine the bistable is in the state shown above. Both the S and R inputs are low and so the output signal is low. A high signal on R would have no effect on the circuit. However, a high signal on S would cause a change to the output of NOR gate A. This, in turn, would cause a change to the input to NOR gate B and change its output.

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A bistable latch circuit – two NOR gates

A B

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If input S now changes to a low signal, this will have no effect on NOR gate A and the bistable will not change state and the output will remain high.

The bistable can only change state if the reset input (R) is high. This causes NOR gate B to have a low output, which causes NOR gate A to have a high output. The bistable has changed state and now has a low output.

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A bistable latch circuit – two NOR gates

S = 0 0 A 1

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1 B R = 1 output = 0 If the reset input (R) now reverted to low, this would have no effect on the state of the bistable circuit.

In effect the bistable circuit ‘remembers’ which of the inputs S or R was last in the high state. This makes it useful in electronic memory.

64 Mb of RAM has over 536 million bistable circuits!

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A bistable latch circuit – two NAND gates

R = 1 A B

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A bistable circuit can also be made using NAND gates instead of NOR gates. A bistable circuit using NAND gates changes state when one of the inputs switches from high to low.

If S switches from high to low this will cause the output from NAND gate B to switch to high.

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A bistable latch circuit – two NAND gates

A B

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Switching S back to high would have no effect on the circuit.

Switching R to low would change the output from NAND gate A which would switch NAND gate B and change the state of the bistable circuit.

Switching R back to high would not effect the circuit and the bistable is reset.

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36 of 39 Contents

Electronic control

Controlling current Electronic control systems Logic gates Bistable latch circuits

Summary activities

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Glossary

      

bistable latch circuit –

consists of two NOR gates.

A simple memory circuit which

input sensor –

The part of a control system which detects changes that the system responds to.

logic gates –

Electronic switches that produce an output signal based on the correct combination of input signals.

output device –

The part of a control system which is controlled by the processor.

potential divider –

A device which converts a change in resistance in the input device into a change in voltage.

processor –

The part of a control system which processes information from the input sensors.

relay –

A switch operated by an electromagnet, which allows one circuit to control another circuit.

37 of 39 © Boardworks Ltd 2005

38 of 39 Anagrams © Boardworks Ltd 2005

39 of 39 Multiple choice-quiz © Boardworks Ltd 2005