Transistors are an essential component in a sensor circuit. Usually transistors are arranged as a pair, known as a ‘DARLINGTON PAIR’. It is very important that you can identify this arrangement of transistors and state clearly why they are used.
A darlington pair is used to amplify weak signals so that they can be clearly detected by another circuit or a computer/microprocessor.

The circuit opposite is a ‘Darlington Pair’ driver. The first transistor’s emitter feeds into the second transistor’s base and as a result the input signal is amplified by the time it reaches the output.

The important point to remember is that the Darlington Pair is made up of two transistors and when they are arranged as shown in the circuit they are used to amplify weak signals.

The circuit to the right shows a single transistor. When the switch is pressed current flows from the 9v to the 0v and also to the base of the transistor. This allows the transistor to switch and in turn, current / voltage flows through the bulb, which lights.

However, there is a potential problem with this circuit. The signal / current at the base of the transistor may be too weak to switch the transistor and allow the bulb to light or it may flicker on and off.

A possible solution is seen to the right. A second transistor is added to the circuit, the circuit is now likely to work as the original signal / current is amplified.

The amount by which the weak signal is amplified is called the ‘GAIN’.

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