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REPLACING THE BULB
WITH A BELL

     Similar to both your light-bulb indicator and home-made electric light is the portable burglar alarm on page 25. Can you see the similarity? Sure you can. All have the same three basic parts: a component that does something, a power supply, and a means of opening and closing the circuit. Also, all three are connected in a simple series circuit. The only difference is that the burglar alarm uses a bell instead of a bulb.

     The burglar alarm provides a neat way of preventing an intruder from opening an inward- swinging door without being heard. It is placed anywhere along the bottom of the door. When the door opens, it pushes against a contact arm. The arm, in turn, closes the circuit, which triggers the alarm.

Experiment 9

A Portable Burglar Alarm
Materials: 2 wood blocks 4” by 3” by 3/4”. 3 nails. Metal strip. 2 round-head screws. 2’ of hookup wire. Doorbell. 6-volt battery.


     FRAME AND CIRCUIT CONTACTS. After nailing the frame together, drive one of the screws part way into the vertical block about 1/2” from the top. Then loop the bared end of a 6” length of wire around the screw, and finish driving the screw in.

     Cut a strip of metal 5” by 1/2”. After shaping the strip as illustrated, pierce a hole in the flat end.