The robust aluminium opening detector for steel doors is a magnetic contact for protecting large (garage) doors. Unauthorised opening is detected and reported to the alarm panel. This opening detector is ideal for fitting to steel and metal doors. Its special construction enables its use on these materials. 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. Its extremely robust aluminium housing protects it from damage even when driven over.