1. ELECTRONICS Learning Objective To know how the thermisistor works
Can describe uses for the Light dependent resistor (LDR)
Can explain the applications of the diode

2. ELECTRONICS Components
Electronic devices are composed of ‘components’ or parts
We will look at three components in particular:
The Diode
The Light Emitting Diode (LED)
The Light Dependent Resistor (LDR)

3. Summary of components we may already be familiar with:
ELECTRONICS
Components
Summary of components we may already be familiar with:
Switch
A switch is a device for completing or breaking circuits.
When a switch is closed current will flow
When a switch is opened the circuit is broken and no current can flow
Switch symbol

4. ELECTRONICS Components Resistor VariableResistor symbol
A resistor is a component that is used to control the amount of current flowing in a circuit.
A ‘variable’ resistor is a component whose resistance can be changed while connected in a circuit to allow different amounts of current to flow ( e.g. a dimmer switch )
VariableResistor symbol
Resistor symbol

5. Diode Now Lets look at some new components The diode
Allows electric current to flow in one direction only

6. Diode The diode symbol Direction of Current
This end represents the ‘positive’ end of the diode
Silver bar at end of diode represents the ‘negative’ end of the diode
This end represents the ‘negative’ end of the diode
The diode symbol
Direction of Current

7. The diode (Forward bias) The diode (Reverse bias)
When diode allows current to conduct it is said to be ‘forward bias’
The diode (Reverse bias)
When diode does not allow current to conduct it is said to be ‘reverse bias’

8. Diode
The diode is a non-ohmic conductor It allows the current to flow in one direction only.
Doesn’t allow the current to flow
(has infinite resistance in the reverse direction)
Allows the current to flow in the forward direction

9. Diode Current (A) Voltage (V) diode
Plotting a current-voltage graph for a diode produces this shape line
Current (A)
Very steep resistance is very small once certain voltage is reached
Voltage (V)

10. Applications of the Diode
Used in Power Supply Units to change from alternating current (a.c) to direct current (d.c) (diodes used this way are referred to as a ‘rectifier’)
Used in circuits to make sure current flows in one direction only (example: bedside buttons in hospitals that connect to the nurses station activating a buzzer)

11. Diode

12. Diode

13. The Light-Emitting Diode (LED)
Diodes that give out light when current flows through them are called Light-Emitting Diodes (LED).
The LED will only allow current flow through it when it is ‘forward biased’
When forward biased the LED emits light

14. The Light-Emitting Diode (LED)
LED uses very small current when emitting light
To prevent damage to the LED, a ‘resistor’ will always be connected in series with the LED
LED Symbol
LEDs

15. LED
For ‘forward bias’ connection the long lead of the LED (Anode) is connected toward the +ve side of battery.
The short lead (Cathode) connected toward the –ve end of battery.
Connections are other way around for ‘reverse bias’ operation

16. Applications of the LED
Used in displays such as calculators, alarm clocks
Used in TVs to show that there is power to the unit
LED lamps ( Torches, Bicycle and Car lights)

17. The Light-Dependent Resistor (LDR)
A resistor whose value of resistance can change as the intensity of the light falling on it changes
If bright light falls on LDR the resistance is low ( larger current can flow)
If dim light falls on LDR the resistance is high ( smaller current will flow)

18. The Light-Dependent Resistor (LDR)
The LDR
LDR Symbol

19. Applications of the LDR
Streetlights. When darkness falls, resistance of LDR increases and used to control switching on of light
Cameras. Uses amount of light entering camera to control camera settings

21. Thermistor
A resistor whose value of resistance can change as the temperature changes
If it is warm the thermistors resistance is low ( larger current can flow)
If it is cold the thermistors resistance is high ( smaller current will flow)

22. Thermistor

23. Applications of the thermistor
Bath thermometers (Baby thing, ask Mr Gamble!)
Anything that senses temperature

24. Thermistor

25. Transducers Is a funny word… Light is the input transducer for a LDR

26. ELECTRONICS Battery Power Supply Unit
Component
Symbol
Thermistor
Battery
Power Supply Unit
Identify the component from the symbols shown
Switch
Resistor
Diode
LED
LDR

27. Key Points ( Fill in the Gaps )
ELECTRONICS
Key Points ( Fill in the Gaps )
A _______ allows current to flow in only one direction
The LED is a diode that emits ________ when in forward bias
A _________ is placed in series with an LED to limit the current
An LDR is a resistor whose value of resistance changes as the intensity of the __________ falling on it changes
diode
light
resistor
light

28. Key words Electronics Component Diode Forward bias Reverse bias Anode
Cathode
Light Emitting Diode (LED)
Light Dependent Resistor (LDR)
Thermistor
Transducer

29. ELECTRONICS
The End