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  1. Describe the action of thermistors and light-dependent resistors and show understanding of their use as input transducers.

A thermistor is a temperature dependent resistor which means that its resistance varies with the temperature of the surrounding. It’s resistance reduces when more heat is applied and vice versa. It is used in fire alarms, where a high temperature, indicating a fire, causes the resistance to reduce and more current to flow, activating the alarm. In an automatic air conditioning system, when the room is warm, the resistance drops to activate the cooling. It’s symbol is:
thermistorAn LDR or a light dependent resistor, where the resistance decreases when light intensity increases. It’s used in burglar alarms, where if a light is turned on, the resistance of the LDR drops and current flows to activate an alarm. It is also used by photographers for automatic shutter control based on the lighting of the surrounding. The symbol for an LDR is:

Both thermistors and LDRs are called transducers since they convert energy from one form to another. Thermistors convert heat energy into sound energy (in the fire alarm example) and LDrs convert light energy into sound energy (in the case of the burglar alarm)

  1. Describe the action of a relay and show understanding of its use in switching circuits.

We’ve learned that a relay uses an electromagnet and switch to control circuits. Now, let’s dive in deeper.
In a switching circuit, the relay is an electromagnetic switch which controls large current with small current.

The relay consists of two circuits- one with a small current and the other with a high current. When the switch is closed, current flows and the electromagnet attracts the iron rocker arm. The arm then rotates about the central pivot and pushes the contacts together. Circuit two now gets switched on. When the switch is open, the electromagnet releases the rocker arm and the contacts separate.


  1. Recognise and demonstrate understanding of circuits operating as light-sensitive switches and temperature-operated alarms using a relay.

Burglar alarm: when a light is turned on, the LDR’s resistance is reduced and a current starts flowing. This activates the relay coil which switches on the alarm.


Temperature-operated fan: When the temperature increases (becomes hot), the thermistor’s resistance decreases, allowing current to flow and activate the relay which switches on the fan.


Notes submitted by Lintha

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