A radio intrusion alarm system utilizing a high frequency carrier signal having an audio subcarrier which may be modulated by two tone signals, the first tone signal causing actuation of an alarm, the second tone signal causing actuation of a time delay means which inhibits operation of the alarm to...http://www.google.es/patents/US3969709?utm_source=gb-gplus-sharePatente US3969709 - Wireless burglar alarm system
