The door opening detector is a magnetic contact for protecting large (garage) doors. Unauthorised opening is detected and reported to the alarm panel. The door opening detector is mounted chiefly on roll-up, sliding, and up-and-over doors. The detector consists of a magnetic contact (reed) and a magnet. These are fitted parallel to each other. The electric circuit of this NC opening detector is normally closed (NC). If the distance between reed and magnet increases, the circuit is broken and an alarm reported. The detector is tamper-protected. The best installation location is in the middle of the entrance. The magnetic contact is mounted on the ground and the magnet on the inside of the door at a distance of about 20 mm. The door opening detector is perfectly suited for surface-fitting. It is absolutely water and dust-proof (IP 68). Its extremely robust aluminium housing protects it from damage even when driven over.