Intrusion detector having a sabotage surveillance device

Abstract

An intrusion detector has a housing with an infrared device disposed therein, an infrared sensor, a detector window provided in the housing wall for the passage of infrared radiation from the external space onto the infrared sensor, an element for focusing the infrared radiation incident through the detector window onto the infrared sensor and having a sabotage surveillance device including an infrared transmitter and an infrared receiver. The infrared transmitter and the infrared receiver are disposed inside the housing and the detector window is substantially transparent to radiation emitted by the infrared transmitter. The sabotage surveillance of the detector takes place by measuring the proportion of the radiation reflected onto the infrared receiver from the inside of the detector window and the radiation transmitted onto the infrared receiver from the surrounding space. The detector may also contain an ancillary detector device such as an ultrasonic device with an ultrasonic transmitter and an ultrasonic receiver. The signal of the ultrasonic receiver preferably has two frequency ranges; one range which is typical of movements in the space under surveillance and the other which is typical of sabotage of the detector. A common evaluation circuit is provided for the ultrasonic section and the infrared section.