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EXPERIMENT 4: Build a Burglar Alarm

THINGS YOU NEED: A few old tin cans. A few pieces of wood, each about 2” square. A few small wood screws. Some thin cardboard. String. Some insulated wire. A transistor radio or an electric bell and a 6-volt lantern battery. Some adhesive or plastic tape.

Let’s build a burglar alarm using a parallel circuit. This simple burglar alarm really works. You can use it to watch over the windows and door in your room, and detect “intruders.” What’s more, it admirably demonstrates the parallel circuit principle that is used in home electrical wiring. Almost a century ago, Thomas Alva Edison, for whom Lewis Latimer worked, innovated and perfected the use of parallel circuits in home lighting, and invented various devices involved.

The heart of the alarm is an ordinary transistor radio (or, if you prefer, an electric bell — we’ll say more about this later). A set of special switches wired in parallel controls the radio. Each switch protects a separate door or window . . . it closes the circuit (and turns on the radio) when the door or window is opened. Because the different switches are connected in parallel, the alarm “goes off’ (the radio sounds) if any switch is “triggered.”

The first diagram shows how each switch is put together. It consists of a pair of metal contacts mounted on a wooden base. Make each contact from a 1/2-inch by 2-inch strip of metal cut from a tin can. Follow the instructions for cutting up a tin can given in the first experiment; be careful of the sharp edges. Bend each metal strip into a crooked “L”. The long leg of each “L” should be about 1½ inches long.

Make the base out of a piece of wood about 2 inches square. Fasten the short legs of the pair of contacts to the base with small wood screws. This is important: Position the “Ls” so that the tops of the long legs press against each other.