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Experiment 11 - The Delay Circuit

Consider the circuit shown. In the circuit diagram you see the capacitor C1 in combination with the resistor R1.

Delay Circuit. (Enlarge)

Circuit diagram of a delay circuit.

This is called a delay circuit. When closing the switch S1, the light-emitting diode LED1 lights up with a delay of afew seconds.

And this is going on in the circuit...

  • Closing the switch S1, the total current flowing through the resistor R1 also flows as a charging current through the capacitor C1. This basically short circuits the base and the emitter of the transistor and the transistor is shut. The LED remains dark.
  • If the capacitor C1 is fully charged, the charging current dries up and the capacitor becomes an insulator. The total current through the resistor R1 now flows to the base of the transistor and the LED lights up.

From the last experiments it is also clear that a capacitor with a higher capacity results in a longer delay.

But not only the capacitor determines the behavior of the circuit, as you can see easily in the following experiment...