Can you just use a simple PIC chip?
Just to clarify our understanding:
1) You have a constant +12V power supply to the circuit and Ground
2) You have a +12V input signal to the circuit that will initiate a timer.
3) The timer, when complete, will interrupt a +12V line supplying power to another section of the overall design
4) You require some analog control over the length of the timer (simple pot, probably)
5) You require some method to reset the circuit, such as a reset button
To do this, you need several "inputs" to the circuit:
1) +12V PS
2) 0V GND
3) +12V SIGNAL
4) +12V INTERRUPTED LINE
Potentially, you could have separate grounds, but that is negotiable.
Mogwai is probably on the right track if you want discrete components. It's cheaper and faster if you know component design instead of microchip programmming. Simply use a switch to toggle a flip flop that initiates a current limited feed into a capacitor. The capacitor voltage is then compared to a reference voltage given by a pot (your timer control). When the comparison reaches "true", the comparitor sends voltage to a transistor which interrupts the +12V INTERRUPTABLE line.
There's more to a good design, of course. You'll want lots of signal conditioning if it is in an automotive environment. Also, pay good attention to your grounds! Do not skimp on components, as a blown transistor is a pain to replace on race day. Etc... etc...
Good luck.
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