Car Alarm Arming Horn 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.

Parts

C1               0.01uF ceramic disc capacitor

C2               100uF  35v electrolytic capacitor

R1                1k

R2                10k

R3                15k

R4                470ohm

D1,3,4         3(1N4004)rectifier diode

D2                red led

U1                555 timer ic

K1                spst 12v automotive relay

source