I need a light curcuit that turns on at different times....

[ciper]

The light is a small 12 volt iincandescent.

In the key off position its off.
In the key accessory position its off.
In the key on position with engine off its on
In the key on position with engine on its off

Seems simple enough but I cant think of how to do it. Think of it as a Key on/Engine off indicator.

The simpler the curcuit the better.

[Legacy777]

actually....that's a little tougher then you think. There's not much in terms of electrical 12v power that changes when the key is in the on position, engine off, and engine on.

I can't think of an easy way to do this with a simple circuit.

I'll let other people chime in.

[vrg3]

I can come up with a circuit that can do it based on the ECU's tachometer output signal. Basically, just set up a retriggerable one-shot timer triggered by the tach signal. Make the decay time approximately a fifth of a second. That way, if it isn't triggered at least five times a second, it shuts off.

But -- what's this for? There are simpler ways to do it if you don't mind a potential malfunction when something else malfunctions. Consider that the alternator charge idiot light on your dash behaves exactly as you describe, except that it also comes on when the charging circuit fails. If that would be okay, just connect one end of your bulb to the "light" terminal of the alternator and the other end to an "ignition" +12v source. You'd be putting your bulb in parallel with the stock warning light.

[Brat4by4]

How about explaining what it is for? That would probably help.

[ciper]

I think this type of light would be very useful for smog tests, it would be what drives the check engine light instead of the ECU.

When getting a smog test they do a visual inspection. One of the visual tests is to make sure the CEL works properly.

[vrg3]

Hehehe... you're sneaky.

Well, you have your answer there. Just disconnect the end of the CEL bulb that's going to the ECU and instead splice it into the wire going from the CHARGE bulb to the alternator. The other ends of both bulbs already go to ignition +12v.

What are you doing that would cause the CEL to be illuminated when the car's functioning correctly?

I'd rather make it so the code doesn't get thrown in the first place, so that the light can continue to be a useful indicator of engine management problems.

[Brat4by4]

better yet, he could hook up a switch that would choose which lead he wants to illuminate the bulb. he could switch between diagnostic or smog.

[ciper]

Its for a car that I have decided to spend the minimum amount of money possible. Id hate for the farking stupid purge valve cel to show up during a smog test.

[vrg3]

Maybe you could just get two or three AT dropping resistors and put them in series between ECU pins F47.6 and B48.2. Would that get rid of the CEL?

[ciper]

What If I get an IDLE switch CEL or a Knock sensor CEL?

You dont understand, this car will have the least amount of work done to it as possible. Its only for fun.

[vrg3]

Oh. I thought you had a car with a CPC CEL and you just wanted it to pass emissions.

I gave you a good suggestion above for just making the CEL go away.