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Transistor Basic Circuits » Electronics » Semiconductor Devices » Transistor » Let's go... » Experiment 7 - The Switch

Experiment 7 - The Transistor as a Switch

Basically, in previous experiments you have used the transistor in a switch circuit. To illustrate this, expand the circuit from the experiment 2 by a potentiometer.

Switch circuit of a transistor. (Enlarge)

Protect Base

Never connect the base directly to +9 volts! It would result in the flow of an excessive current and immediately destroy the transistor.

Therefore always protect the base by a resistor (R1)!

Circuit diagram of a switch circuit.

When turning the potentiometer P1, the LED lights up very brightly for a long time before she goes out relatively quickly. A small base current is sufficient to fully switch through the transistor. If the base current decreases below a certain level, the transistor shuts rapidly.

The circuit shown is also known as a common emitter circuit. Wherever a transistor is used as a switch, this circuit is applied.

What is the difference to an amplifier?