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.
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