Key operated vehicle theft-prevention circuit - has resistive key in bridge circuit connected to discriminator in interrogation circuit

摘要

The anti-theft system for a vehicle uses a special resistance key (5) for a door lock, connected to an interrogation circuit (3). The key operates a conventional cylinder mechanism (2). The tip of the key closes a switch (6) controlling the interrogation circuit. The key carries resistances (7,8) which are connected by a set of contacts (9 to 14) to the interrogation circuit. This includes a bridge circuit (39) which is combined with a discriminator (25) so that in the event of an incorrect resistance being introduced into the bridge, an alarm activating signal is obtained at an output terminal (A). The window discriminator is interconnected with diodes (36, 37, 43, 44) for blocking peak voltages at two inputs (28,29). The interrogation circuit is also connected to a resistance capacity system with two capacitors (34,41) and two resistances (38,45) for blocking fault signals.