How our alarm works

Here's how our motion-triggered alarm works. It uses a mercury switch like the kind used in car alarms. The mercury switch is connected to terminals W3 and W4. When the switch is disturbed, it grounds the trigger input, which causes the output to go high. This turns the buzzer on.

Motion-triggered alarm

The alarm will continue to sound until the internal flip-flop is reset. We do it by placing a magnet within the range of the reed switch. Closing the switch grounds the reset pin and turns off the buzzer by making the output go low. It's that simple. A suggested breadboard layout is shown in the following image:

Motion alarm breadboard