Car wiring...

[Dr. kitteny berk]

Tiny bit off topic, I know, but as wifey was a mong earlier and left the sidelights on on the car, resulting in a needing a jump start...

I'm thinking wiring a buzzer between the light switch and the ign live on the car with a diode to stop the ign current resulting in lights always being on, end result should be a pleasingly annoying buzz when the ignition is off and the lights are on.

Am I thinking right?

[Dog Pants]

In basic terms, maybe. Is there power to the ignition circuit when the car is running, or is it only used to turn the engine over? If so then as long as the potential difference between the two circuits doesn't fry the diode (a resistor will fix this if it does), and the current is going the right way, then you'll have power. Fiddling the resistor value should make sure you don't bleed too much voltage off in parallel to dim your lights. Since the ignition circuit switches a relay that's going to AC, which is good because that's what you'll need to run the buzzer. Your lighting circuit might not be though, which is a problem. A bigger problem is that diodes don't work the way you want them to with AC - they'll let half of the wave through and block the other half. Lastly, you need to hope that the AC is alternating at an audible frequency (fair possibility, you're looking at 20Hz to 20KHz, mains is 50Hz) or else you wouldn't hear the buzzer even when it was running.

Idea's sound though, my car does it.

[Dr. kitteny berk]

I'm sure I can get ign live without a problem, and a buzzer will take 12vdc (possible issue just came to mind with charging voltage (~14v) from alternator, but shouldn't hurt as the buzzer will only work when the engine is off.)

[Dog Pants]

Ah yes, I forgot that modern technology means they can design buzzers to take DC. The voltage isn't necessarily a huge issue, just put it in series with a big fucking resistor (the amount of current I imagine you'll get from the alternator will warrant something substantial). Bone up on Ohm's Law to work it out; V=IR.

[Dr. kitteny berk]

I'm not sure it's that essential, as there'll be no voltage difference while the engine is running, so there'll be not much excess current flying about, should just need a decent enough diode to stop voltage leaking the wrong way.

Only time there'll be a voltage difference across the buzzer will be if the lights are on and the ign is off, or the ign is on an the lights are off (but surely a decent diode will stop that?)

I think.

[buzzmong]

Wouldn't it just be easier to wire up a PNP transistor to the buzzer in parallel to the light circuit and use the ignition lines (from the key) to control the base lead?

PNP's work by turning on when the base goes lower than the emitter, so in this case, the buzzer would be off while the ignition is on (base is high).

[Dr. kitteny berk]

Wouldn't it just be easier to wire up a PNP transistor to the buzzer in parallel to the light circuit and use the ignition lines (from the key) to control the base lead?

PNP's work by turning on when the base goes lower than the emitter, so in this case, the buzzer would be off while the ignition is on (base is high).

Not sure it'd be easier, as you'd be replacing the bodgy goodness of relying on the non-live-live to act as ground, replacing it with a transistor to switch stuff.

Sure, it'd be waaaay more elegant, but fuck elegance, I'm thinking KISS.

[ProfHawking]

Yeah, I'd say you could get away with keeping it simple.

Assuming the buzzer has a high resistance, it should work ok. I can't imagine it would be a problem unless its a modern car with some other switching going on for the ignition, the current coming through the buzzer won't be enough to trigger ignition.

Drawything:

[Dr. kitteny berk]

Yep, that's exactly what I had in my head, glad it makes sense