You saw in our last Project, see Capacitor section, how a capacitor can store and discharge electricity. You probably noticed that capacitors take a certain amount of time to charge and discharge. Some of you were probably thinking that this fact could be put to some good use in a timer circuit of some sort, and it happens you're right.
This Project will show how the charging time of a capacitor can control circuit operation.
When you finish the wiring connections, out the power ON. Listen carefully to the Relay. A few moments after you put power ON, you'll hear a faint "click from the Relay as it operates. Can you guess why it waited to operate? (Hint — take a close look at the schematic.)
The delay happened because of the two 100uF capacitors in parallel. When they finally built up enough charge, enough current flows to the Transistor to allow the Relay to operate. Can you think of other circuits or devices that could make use of the time it takes capacitors to charge and discharge?