MK24 Reed Switch Door Sensor

Detect a door opening with a magnet, a tiny glass reed switch, and one Arduino digital pin — the building block of every burglar alarm.

Difficulty: Breadboard. Estimated build time: about 20 minutes. Estimated parts cost: about US$2.50. 5-part bill of materials with live supplier pricing. A magnetically-triggered door sensor built from the Standex MK24-A-2, a glass-encapsulated reed switch about 14 mm long.