This is a simple alarm that will beep when someone enters the door in your bedroom. It is cheap and easy to make. It will beep as loudly as you want, depending on the buzzer you use. It is also very easy to deactivate. This is an advantage if you just want something to give you a heads up when someone enters your room. It can be a disadvantage if you want an alarm that will alert the whole neighborhood. After all, an intruder could disarm it as easily as you could.
Instructions
1. Get your supply. You want a power supply with the same voltage rating as your buzzer. You want the same or higher amperage rating. For example, if you buy a buzzer that needs 9 volts of power at 100 milliamps, you want a power supply that provides 9 volts of power and at least 100 milliamps. If the power supply provides more voltage, you risk damaging the buzzer.
2. Attach one terminal of the power supply to the buzzer. Use copper wire and solder for a permanent connection.
3. Put the buzzer in a box or case to protect it. Figure out where you are going to position it in the room. It should be somewhere where it will not get stepped on accidentally.
4. Cut two long wires. Attach them to the remaining free terminals. The one attached to the power supply should be long enough to reach the side of the door jam. The one attached to the buzzer should be long enough to reach the door. Wire them along the ceiling or wall to the door. Keep the wires off of the floor. Leave a few feet of slack.
5. Solder one wire to a small iron plate. It should be at least a few inches squared. Tape or glue the plate to the bottom corner of the door.
6. Solder the other wire to the neodymium magnet.
7. Place the neodymium magnet right in front of the door. Place it just far enough away so it does not latch onto the iron plate. As soon as the door is opened, the neodymium magnet will stick to the iron plate. This will complete the circuit.
Tags: power supply, iron plate, neodymium magnet, alarm that, alarm that will, enough reach, long enough