In movies you must have seen that the alarm goes on the moment an intruder opens a door. In this experiment, you can make it for yourself.
Things Required:
Connecting wires with each end stripped 1½ inches bare of insulation
Simple buzzer circuit
5 inches × 3 inches cardboard
Pair of scissors
Metal paper clip
Tape
Two metal thumbtacks
String
Small square piece of paper
Directions:
Cut out a strip of cardboard (removed from a packing box) that is about 5 inches long × 3 inches wide. Push two metal thumbtacks into the cardboard. The tacks should be about 2 inches apart. Wrap a length of conducting wire around the shaft of each tack.
Unbend a paper clip into an “S” shape. Slip one end of the clip around the base of one tack. Adjust the bend in the clip to ensure that there is enough pressure to keep the far end of the clip pressed down on the other tack.
Slip a small square of paper between the clip and the tack. The paper will be held in place by the pressure of the bent clip.
Tape a string to the paper. Tie the other end of the string to a doorknob. Attach the conducting wires to a series circuit that contains two 1.5-volt cells and a 3-volt buzzer.
Tape the cardboard setup down so that it may remain secure. Now invite a friend to open the door. BUZZZZZZZZZZZZZ.
This Is What Happens:
The paper that was inserted between the clip and the tack acted as an insulator. This insulator blocked the flow of electric current. When the paper was yanked away, the clip and the tack made contact. When the circuit became complete, charges flowed and the alarm sounded.
