A household alarm circuit, when switched on during night hours, responds to the touch or proximity of a human body to the house door knob or lock by sounding the existing door bell or chime. The alarm circuit utilizes the low voltage supply for the door chime and has a proximity sensing lead for connection to the striker plate of the door latch. When the door knob or lock is approached or touched, the alarm circuit repeatedly sounds the door chime. The alarm circuit includes a rectifier for connection to the AC door chime supply, a transistor cut off by a potential change transmitted to its gate by the sensing lead, an SCR switch closed by the transistor, and a relay in series with the SCR switch with a contact for closing the door chime circuit.

An intrusion alarm is encased within a housing hung from a doorknob by a conductive strap which makes electrical contact with the doorknob apparatus. A monitoring circuit within the housing is connected to the hanging strap for sensing capacitance changes corresponding to touching of the exterior portion of the doorknob apparatus by a would-be intruder. Alarm means within the housing are activated by the monitoring circuit to generate a local audible alarm or an FM transmission signal sent to a remote receiver or both. A latching circuit maintains the alarm means in an activated state after the capacitance coupled to the doorknob apparatus has been removed, and switch means on the exterior of the housing are provided for resetting the alarm to a non-activated state to permit sensing of a subsequent capacitance change. The alarm further includes means for temporarily disabling the alarm device for a predetermined period of time during which it may be touched without activating an alarm signal. A status indicator is included to provide a flashing signal when the device is actuated and the batteries are supplying power.

An electromagnetic intrusion system is disclosed wherein a sensor having single antenna radiates a sensing field when driven by two independent oscillators having frequencies offset from each other. The oscillators are closely coupled to the antenna and are effected by changes in antenna impedance as results when an object enters the sensing field so that their frequencies deviate. The response of the driving signals from each oscillator and the frequencies thereof are adjusted so that the frequency deviation or pulling of each of the oscillators is out of phase with respect to the other at the selected maximum range, whereas the frequency deviation and therefore sensitivity is dimished in the immediate vicinity of the antenna. The difference frequency is applied to a detection circuit which produces an output in the event of a shift in the differential frequency. The latter circuitry may include a local oscillator, the output of which is mixed with the frequency difference output in two channels. The output of the mixer in one channel is shifted in phase with respect to the output of the mixer in the other channel by 90.degree. at the mixed frequency. A sampling phase detector compares the two channel frequencies and produces an error voltage which varies in polarity depending upon whether the differential frequency is lower or higher than the local oscillator frequency. This error voltage may be used to vary the local oscillator so that no error voltage is produced in the event that an object does not enter the sensing field. Upon entry of an object the error, voltage may be utilized to modulate a transmitter in accordance with a predetermined code representing the sensor. This transmitter may utilize the same antenna, as does the sensor, to transmit a coded signal to a receiving station at which it is demodulated to indicate the particular sensor, out of many, which has detected an object. Inasmuch as each sensor has a separate code, a single receiving channel at a station remote from the sensors may be used to monitor several sensors.

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.

An intrusion detection unit comprises a compacitance having an "active" field, the coupling of which is significantly increased when an intruder comes into conductive relation with a doorknob. The unit is hung on the inside doorknob and is so arranged that the doorknob is the transmitting element of the capacitor, while a separate plate is the receiving element of the capacitor. The capacitor field is maintained actively charged by a battery driven oscillator which operates at a substantially uniform frequency and amplitude. The intrusion detector circuit is complete in itself and is not externally grounded by the intruder. The signal receiving portion of the circuit incorporates a square law amplifier, and also has means for adjusting the reference level to which the signal is compared.