Car Alarm Arming Beep Canceller

It's a great convenience that most modern cars come with a built in alarm, however it is nothing but noise pollution that the horn sounds when the alarm is armed. Disconnecting the alarm system from the horn relay will eliminate this, but prevent the horn from sounding in the even of an actual alarm. This circuit serves to silence the arming beep yet maintain the alarm by introducing a small delay into the signal. It sits between the alarm and horn relay. The alarm must provide a constant horn signal for at least 3 seconds before the horn relay is activated. That way the quick `beep` will never activate the horn relay, while the constant alarm signal will.
Car Alarm Arming Beep Canceller - schematic

C1 1 0.01uF Ceramic Disc Capacitor C2 1 100uF 35V Electrolytic Capacitor R1 1 1K 1/4W Resistor R2 1 10K 1/4W Resistor R3 1 15K 1/4W Resistor R4 1 470 Ohm 1/4W Resistor D1, D3, D4 3 1N4004 Rectifier Diode D2 1 Red LED U1 1 555 Timer IC K1 1 SPST 12V Automotive Relay MISC 1 Board, Wire, Socket For U1, Case This circuit is by bob and was originally posted to the forum as Need Circuit Project to Improve Remote Car Alarm.

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