4M KidzLabs DIY set Magnetic Intruder Alarm

The disc is made of steel, a magnetic metal which is attracted by the magnet. When the door is closed and the triangles on the magnetic holder and base are aligned, the disc is pulled to one side of the eyelet area by the magnet. When the door is opened, the magnet moves away from the base and the pull from the magnet is no longer strong enough to hold the disc. The disc rolls down into the bottom of the eyelet area and rests across the two terminals. This acts as a switch, completing the circuit and allowing current to flow through the buzzer.