Intruder alarm system using radio link - detects phase offset of received signals to prevent alarm operation due to parasitic vibrations

摘要

The system has spaced detectors controlling respective transmitters, for sending a coded alarm signal to a central receiver. The receiver has a code generator (72) providing a code identical to that provided by the transmitter and a code comparator (73) comparing the two code signals. The output of the code comparator (73) is fed to a threshold circuit (74) in turn coupled to a memory device (75), for allowing the alarm to be operated at the end of the code when the difference or the phase offset between the two code signals is below a preset threshold. Pref. the detectors are spaced at different points within the protected locality, responding to parasitic vibrations to supply phase offset signals, with the code comparator (73) receiving the signals after amplification and preventing operation of the alarm. The memory device (75) comprises a D.type multivibrator with a clock input (Ck) coupled to a timer (76) provided by a monostable multivibrator. The system is insensitive to parasitic vibrations caused by explosion etc.