Abstract: An intrusion alarm having an oscillator which is turned off by the body capacitance of a would-be intruder operates in the frequency range of 17-65 MHz. At this frequency, reliable discrimination between body capacitance and stray capacitance is possible and therefore sensitivity is increased while false alarms are reduced. Battery life is increased by providing a high resistance load in the stand-by mode to reduce battery drain to 500 microamperes or less. Latching and nonlatching embodiments are disclosed.

Abstract: This intrusion alarm apparatus is intended to be suspended from an inside doorknob and plugged into an A.C. power socket at the premises to be protected. A test switch and indicator lamp enable the user to determine (without triggering an alarm) whether the plug has the correct polarity in the power socket. The alarm signalling device in the apparatus or the indicator lamp is activated by the added capacitance of a person or an extraneous metal object touching the outside doorknob, depending upon whether the switch is in the "test" or "on" position.