Theft indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Security devices and alarms
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The proposed device consists of a miniature VHF transmitter, which is powered by a rechargeable battery, and an FM radio receiver with a prefix in the car owner's apartment.
A miniature transmitter (Fig. 7, a) is installed before parking the car at night in such a place that it is not immediately detected by the hijacker. You can make an FM receiver yourself according to any known scheme or use an industrial one that has a VHF band.
(click to enlarge)
When the transmitter radiates and is in the "radio visibility" zone of the receiver, the hissing disappears at the latter at the output, and the transistor VT2 (Fig. 7, b) is closed, since there is no signal at the output of the FM receiver and there is no voltage on the secondary winding of the transformer T1. Transistor VT1 is open, relay K1 is on. When the car drives away a few meters, the power of the VHF transmitter becomes insufficient to trigger the receiver's squelch, resulting in noise.
An alternating voltage appears at the output of the transformer T1, which is rectified by the VD1 diode and filtered by the capacitor C8. VT1 opens, relay K1 turns off and the normally open contacts turn on the audible alarm.
If a hijacker has detected a VHF transmitter and disconnected the power, the receiver will still beep.
Author: N. Yakovlev, Nizhnevartovsk
See other articles Section Automobile. Security devices and alarms.
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