Output devices
Digital Output Devices
Analogue Output Devices
Device
Energy Change
Device
Solenoid
electrical to kinetic
Motor
electrical to kinetic
Buzzer
electrical to sound
Loudspeaker
electrical to sound
LED
electrical to light
Relay
electrical to kinetic
7-Segment display
electrical to light
Lamp
Energy Change
electrical to light
The Light Emitting Diode (LED)

LED (Light Emitting Diode) converts electrical energy into light, but it will only do so
when connected the correct way round.
electron flow

A resistor is always placed in series with an LED to prevent it being damaged by too large a
current passing through it.
Example - Calculating the resistance of the series resistor
In the circuit opposite the LED operates at 2 V and 10 mA.
Calculate the size of the series resistance required
VR = Vs - VLED = 9 – 2 = 7 V
R=
=
Input devices

Many input devices are energy changers; they convert some form of energy into an
electrical signal. (Note: These are all analogue devices.)
Microphone
sound to electrical
Thermocouple
heat to electrical
Solar cell
light to electrical

Some input devices are resistance changers
o The resistance depends on some external factor (for example, heat, light or
pressure)
o (Note: These are also analogue devices.)
o These include thermistors, LDRs and variable resistors.
Light Dependent Resistor (LDR)


The LDR is a type of resistor whose resistance changes depending on light level
As light level increases, the resistance of the LDR decreases
o (Light Decreases its Resistance)
Thermistor

The thermistor is a type of resistor whose resistance changes depending on temperature
o For most thermistors, as temperature increases, resistance decreases
o (same response as LDR has to light level)
Transistors
1. The n-channel enhancement MOSFET



2. The npn Transistor
Both of the above transistors act like an electronic switch.
The npn transistor switches on at around 0.7 V. The MOSFET switches on at around 2 V.
When a transistor switches on, the output device will activate.
Explaining the operation of transistor switching circuits
Example – temperature warning light
A transistor switching cicuit is designed for a car dashboard to warn drivers when the engine
temperature gets too high.
Explain how thew LED lights when the temperature gets too high.
Solution





When the temperature of the engine increases, the resistance of the thermistor decreases.
This causes the voltage across the thermistor to decrease.
As a result, the voltage across the variable resistor increases.
When the voltage across the variable resistor reaches 0∙7 V, the transistor switches on
This causes the LED to light
Logic Gates

There are different types of logic gate, depending upon what the gate is needed to do.
OR gates
An OR gate will give a high output if any of the inputs is high. In
other words, there only needs to be an input in A OR B for
there to be an output at Q.
This is the same as having two switches in parallel
AND gates
An AND gate will give a high output only if all of the inputs are
high. In other words, there is only an output if there is an input
in A AND B.
This is the same as having two switches in series
NOT gates
A NOT gate is slightly different because it has just one
input. It will give a high output if the input is low. This
could be represented by a simple lighting circuit with a
push-to-break switch: if the switch is pressed then the
lamp will turn off. NOT gates are often used in emergencystop buttons on machine tools.