Get a positive door trigger without an problems from the interior light delay circuitry in the ARC.
I have finally finished installing my car alarm and everything works as it should. As you would expect of course (being that I did it)
Anyway enough self-congratulation, whilst installing I came accross something that I thought other AlfaOwners maybe interested in.
The door lock/catch mechanism in the door contains the door position detecting switch (open or closed). Which in itself is not particularly strange, however to complicate matters further (as Alfa tend to do) the switches operate in a reverse manner to what you would expect. The switch supplies a ground signal when the door is closed!
Normally you would expect a car to supply ground when the door is opened. So this threw me off for a bit as the alarm would be triggered when the doors were closed and not when the doors were opened by a potential theif or perhaps me having forgotten that I have now installed an alarm!
So having really only a small amount of experience with electrics, mainly 240v appliance repairs and auto electrics, I immediately thought of how I could do it with relays. Some relays (mine were Bosch) have a Normally Closed (sometimes referred to as a 87a (pin 87a)) and a Normally Open contact (pin 87). Of course this lead to a swift flattening of the battery because the relay coils were energised all the time the doors where shut!
Having realised that I am going to have to read up on electronic circuitry/logic gates I decided to take the easy way out and ask on an online electronics forum!
With the help of the people online we came up with nice and simple DTL OR gate, a copy of which is attached.
To use the circuit diagram supplied you will need to find the door wires at the ARC (Alfa Romeo Control) and tap into them. These will connect to the circuit where the door switch is in the diagram. I used a circuit/bread board to keep it neat but you could get away without it.
I also must point out that this will only work if the alarm your installing has a positive trigger wire and draws next to no current through the circuit. Mine was such an alarm and I am sure that almost every alarm worth anything uses this system. Otherwise they would risk blowing peoples door switches by drawing too much current!
DO NOT try putting a positive current direct to your door switches!
They are a reed switch with a magnet if positive is supplied they will fuse together and melt causing catch failure. The doors will not shut anymore!
They will just "bounce" back open. Make sure you put the step down resistors in the circuit and do not make them anything less than 10K Ohm. I think that anything higher than 100K Ohm would stop the alarm from triggering but you could try it out.