This is for a waste-vegetable-oil diesel conversion. Before shutting down, you have to switch the engine over to "regular" diesel fuel and run it for about thirty seconds to clear the wvo out of the injectors and such, so they don't get gummed up. My buddy doesn't want to have to sit in the car after parking it if he forgets to switch over to diesel a block or two before parking and shutting off the engine; or even worse, forget altogether and end up mucking up the engine. So he wants a circuit that will cause the engine to switch over to the diesel line automatically and run for an extra half minute or so after he turns the ignition key off.
Here's a block diagram of my initial idea for the part of the circuit that will cause the engine to run on briefly after turning off the key:
keyed 12 volts | +------------------, | | | ___|___ | | | | | relay |---->to engine | |_______| | | | | | c/ | | '----+-----+-----| npn | | | --- \\ e\\ --- / | | \\ | | / | | | | '-----'-------'-----gnd And use cap and resistor values to give the right amount of run-on time. Obviously it works by maintaining some voltage in the cap for a while, until the resistor (and base current) drains it low enough to where it won't hold the transistor on.
As for causing the plumbing to switch from wvo to diesel when he turns the key off, we can accomplish that by using a 12 volt solenoid in the wvo plumbing line, so that when the operator turns the key off, the solenoid drops out and the engine's plumbing reverts to the diesel fuel line.
Any feedback? I think there might even be a simple circuit that would do the job with just a relay and cap.