The proposed device consists of a miniature УSW transmitter, which is powered by a battery, and FM radio with the prefix in the apartment the owner of the car.
Bodypack transmitter (Fig.7,a) is set before Parking the car in night in such a place that it is not immediately found the hijacker. FM receiver can be easily made by any well-known scheme or use industrial, having УSW-range.
(click to enlarge)
When the transmitter emits and is located in the "range" of the receiver, the last output hiss disappears, and the transistor VT2 (Fig.7,b) is closed, so as at the output of FM receiver no signal and the voltage on the secondary winding the transformer T1 is absent. Transistor VT1 is open, the relay K1 is turned on. When the car drives off a few meters, power УSW-transmitter becomes insufficient for actuating the squelch of the receiver, which leads to noise.
The output of the transformer T1 AC voltage appears, which rectified by the diode VD1 and filtered by capacitor C8. VT1 opens, the relay K1 off and normally open contacts includes an audible alarm.
If the hijacker found УSW transmitter and disconnect the power receiver all still beeps.
Author: N. Yakovlev, Nizhnevartovsk