1. Designing Electric Circuits

2. Designing Circuit
WALT: Understand various components and design my own circuit
I must be able to identify the uses of the electric components in my box and draw the circuit symbols (including LED)
I should be able to design a range of circuits using them – with some help
I might be able to do this on my own

3. Question: In a hair dryer, the heater and the fan are usually connected in a series circuit. Suggest why this is a useful safety measure?

4. Prep
Use pages 291 and 304 to make a table showing the main circuit symbols we’ve used and their uses.

5. LDR (Light Dependent Resistor)
Has low resistance in bright light
and high resistance in low light
Used in security lighting
Draw a sketch of a graph to show the relationship between resistance and light intensity with an LDR

6. LED (Light Emitting Diode)
Give out a very bright light when only a small current flows
Used as little red bulbs to indicate electronic devices are on.

7. Thermistors
Thermistors detect temperature and are designed to decrease the resistance in a circuit as temperature increases
They are used in fire alarms
Draw a sketch of a graph to show the relationship between temperature and resistance with a thermistor

8. Can you remember what a relay switch is?
Uses a small current to turn on another circuit that has a large current.
Used in car ignitions

9. Your tasks 1. Relay switches – the car ignition
Set up a circuit using a reed relay that has a bulb and push switch in the primary (smaller) circuit and a motor in the second circuit.
Draw the circuit diagram and describe how it works
LDR – Set up a circuit to test your theory that LDRs have low resistance in bright light. Use a torch and an ammeter. Draw the circuit diagram and explain what you observed.
SPDT switch
Design a circuit that switches a light on OR a buzzer on, using an SPDT switch.
Draw the circuit diagram
4. Design a circuit of your choice – Design a circuit with a purpose. Draw the circuit and explain it.