A Friend said:
Free vehicle immobilizer design
Get a load of this guy!
http://home.clear.net.nz/pages/dandavies/index.htm
Another concept I've seen is to include a small window comparator circuit
that if 'out-of'window' will partially add a series resistor to the ignition
coil. So if a thief tries to start the vehicle, it appears to be trying to
start, but won't actually fire. So hopefully the thief assumes it's a
rubbish vehicle and moves on. The resistor (23K) that brought the
comparator in-window was mounted inside a 5-pin audio din plug - the socket
looks pretty innocuous, but who carries a 23K +/- 2% around?
No doubt one of many ideas ...
hth
Neil