I designed my own car alarm last year using a 16F84 PIC as the brain. I had a small but very annoying problem with my design.

For the door sensor I used the existing door switch, the non grounded side was connected to a relay which switched +5v to the PIC. When the door was closed there was 12v going to the relay, and when the door was open the relay was grounded.

The problem with this was having the door sensor plugged into the circuit killed the reception to the radio. I know this sounds stupid, but its for real.
Unplug it = radio good, Plug it in = radio bad.

I need new design where a transistor or something similar has in input of 12v (door switch) which switches an output of 5v (into PIC).

Im just new to this forum, I hope to have a good time here.

Thanks peops.

 

I wonder if the PIC isn't radiating or conducting some RF and overloading the radio. If the radio fails when the PIC is installed and operating only while connected to the door then maybe a small capacitor from the input to ground - or a choke in series with the input might help prevent RF from leaving the PIC. Might do the same for the power supply or other leads.

__________________
stevez