This is a plan for a very simple burglar alarm. When the door is opened, it will close the circuit, causing a buzzer to sound. It has the advantages of being easy to put together and cheap to assemble. On the down side, it is not very secure, since it can be easily disconnected. It makes a great basic electronic project or temporary safety measure.

Instructions

Buy a buzzer and a power supply. A power supply is a plug-in adapter that takes AC current from your wall socket and converts it into DC current for your alarm. Find a power supply that has as much or slightly less voltage than the buzzer requires, and as much or more amperage. For example, if your buzzer is rated as 9.2 volts and 25 milliamps, a 9-volt power supply with 50 milliamps would be fine, but a 9.7-volt power supply could damage it.

Connect one terminal of the power supply up to one terminal of the buzzer. Either solder them directly together or use a length of wire to connect them.

Protect the buzzer and power supply. Ideally, you should mount them in a box with a hole for the cord to go through. In a pinch, you can just wrap them in a layer of electrical tape.
Cut two long strands of wire. They should reach from the power supply to the door with at least a few feet of extra give.

Solder the wires to the trigger mechanism. You will need a small, rectangular iron plate, about two inches on each side, and a neodymium magnet. This alarm has a magnet and a small iron plate which together work as a switch. When the door is opened, it will cause the magnet to latch onto the iron plate, completing the circuit and turning on the alarm. Solder one wire to the iron plate, and solder the other one onto the neodymium magnet.

Connect the free ends of the wires to the circuit. Solder one of them onto the free terminal of the power supply and the other to the free buzzer terminal. Touch the magnet to the iron plate. Plug in the circuit. The buzzer should go off. Unplug the circuit again.

Attach the iron plate to the foot of the door with tape. Place the magnet right in front of the door. It should be positioned so that, when you open the door, it will latch onto the plate, turning on the alarm. Plug it in and try it out.

Article Source:   www.eHow.com

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