Simple Output Devices

TASK

1. Collect a set of Output Device cards.

2. Match each symbol to the correct name, energy change and type (digital or analogue).

3. Get it checked.

4. Complete the following table with the correct information:

Symbol NameEnergy Change

Device Type

Symbol Name Energy Change Device Type

M

bulb electrical light analogue

motor electrical kinetic

analogue

buzzer electrical sound digital

solenoid electrical kinetic

digital

Symbol Name Energy Change Device Type

loudspeaker

electrical sound analogue

relay electrical kinetic

digital

Relay Switch

Relay

A relay is a switch controlled by an electromagnet.

A coil of wire:

• carrying a current = magnet

• with no current = non magnet

Symbol:

Relay

M

230 Va.c.

Example of Relay Use

A high voltage mains motor is switched on when switch S is closed.

The small current in the 9V circuit makes the electromagnet become magnetic.

This closes the switch and starts the motor.

9 V

S

Reed Relay

The reed relay uses a permanent magnet to open or close a switch.

This type of relay is used in burglar alarm systems to detect when a door is opened.

: :. .

: :. .

to alarm

S

magnet

Experiment

When the door is closed the magnet is close to the reed relay and switch S is open.

When the door is opened, the magnet moves away from the relay, causing switch S to close setting off the alarm.

Solenoid

A solenoid is a coil of wire surrounding a metal rod.

It is an electromagnetic device that moves a metal rod backwards and forwards.

Examples of Solenoid Use

• car central locking

• safe locks

• steering on radio controlled cars

• railway points

Light Emitting Diode (LED)

LED

Symbol:

Energy Change: electrical light

Type: digital output device (only ON or OFF).

Wiring up an LED

An LED only conducts when it is connected the proper way around in a circuit.

LED lights

(FORWARD BIAS)

LED does not light

(REVERSED BIAS)

electrons flow into point of

triangle to light LED

resistor in series

An LED is damaged if too large a current passes through.

A series resistor limits the size of the current, protecting the LED from damage.*** ***

Q1. (a) Which LED’s in the following circuits will light?

R

LED 1

R

LED 2R

LED 3

RLED 4

(b) What is the purpose of the resistor R in the above circuits?

(c) When an LED lights it is said to be biased.

LED’s are wired this way.

(d) When an LED does not light it is biased.

forward

reverse

protect the LED from damage

1 and 3

Calculating Size of Resistor

The resistor and LED are in series.

So the voltage across the resistor and the voltage across the LED must add up to give the supply voltage.

VR + VLED = VSUPPLY

VS

0 V

VR

VLED** not on data sheet **

x

÷

Example 1

An LED can withstand a maximum current of 20 mA at 2 volts.

Calculate the minimum value of resistor required when the LED is connected to a 5 V supply.

V 2VLED

25VR V 3

mA 20I A 10 20 -3

?R

IV

R

IV

R R

3-10203

Ω 150R

V

I R

R IV

Example 2

An LED can take a maximum current of 18 mA at 2.2 V.

Calculate the minimum value of resistor required to protect the LED when it is connected to a 6V supply.

V 2.2VLED

2.26VR V 3.8

mA 18I A 10 18 -3

?R

IV

R R

3-10183.8

Ω 211R

Q1. The maximum voltage allowed across an LED is 2.3 V and the current through it must not exceed 10 mA.

The LED is connected to a 5 V supply.

Calculate the value of the resistor R connected in series with the LED.

270 Ω

Q2. An LED takes 10 mA and 1.5 V to work properly.

What value of series resistor is required if a 6 V battery is used.

450 Ω

Q3. An LED and resistor are connected in series to a 9 V supply.

The maximum voltage across the LED is 1.8 V and the current through the LED must not exceed 12 mA.

Calculate the value of resistor required to protect the LED.

600 Ω

d

g

7 Segment Display

7 rectangular LED’s are arranged as follows to give a 7-segment display.

a

b

e c

f The number 3 is displayed by lighting

segments a, b, c, d and g

Energy Change: electrical light

Type: digital output device (segment ON or OFF).

Experiment 1 – Making Numbers

Using a 7-segment display, display each number from 0 – 9 in turn completing the table below.

Digit Segments Lit

0 a b c d e f

1 b c

2 a b g e d

3 a b c d g

4 f g b c

5 a f g c d

6 a f e d c g

7 a b c

8 a b c d e f g

9 a b c f g

Experiment 2 - Making Letters

Make as many letters as you can on the 7-segment display completing the following table.

Letter Appearance Segments Used

A a b c e f g

Which Output Device?

Selecting Output Devices

Here is a list of output devices:

buzzer, lamp, motor, relay, solenoid

Which output device could be used to:

(a) indicate required cooking time for oven is complete

(b) move conveyor belt at supermarket

(c) stop and start movement of conveyor belt at supermarket.

buzzer

motor

relay

Q1. Give a suitable output device for each of the following situations:

(a) an audible warning when an incubator become too hot or cold

(b) a power on indicator for a stereo

(c) switch on a high current circuit using a small current

(d) indicator of which TV channel is selected

(e) to push chosen sweets to hatch in a vending machine

(f) run a conveyor belt around a bottling factory.

buzzer

LED

relay

solenoid

7-seg.

motor

Binary Numbers

Binary numbers consist of a series of 0’s and 1’s.

Consider the binary number 1101.

1 1 0 1

means 1 lot of 8

means 1 lot of 4

means no lots of 2

means 1 lot of 1

So we have as a decimal number: 8 + 4 + 0 + 1 = 13

8 4 2 1continue to

increase (double)

just in same way as we represent

decimlas by

H T U

7 8 9

** You must know the binary for numbers from 0 to 9 **

8 4 2 1

Decimal Binary

0 0 0 0 0

1 0 0 0 1

2 0 0 1 0

3 0 0 1 1

4 0 1 0 0

5 0 1 0 1

6 0 1 1 0

7 0 1 1 1

8 1 0 0 0

9 1 0 0 1

Q1. Copy and complete the table of decimal and binary numbers:

Decimal Binary

00102

01117

111115

1001018

1011123

Q2. What decimal number is represented by the binary number:

(a) 0100 (b) 10014 9

Relay