DC Voltage level sensing

comparator. Manually adjusted relays were once used but electronics be more reliable, assuming it's designed to handle car electric nasties. You could also use the charging light.

NT

Get a TL431 datasheet. Buy an TL431ACLP, in TO-92 package. Add three resistors and a p-channel MOSFET, maybe an IRF9540N in TO-220, and you're good to go.

Many cars cut power to the cigarette lighter when off. I would simply dete ct that if your car is one that works this way.

If you are making this for a general application, you can use a 12 volt reg ulator, a PNP transistor and a diode. Connect the regulator normally with the diode base and emitter across the input (battery) to output, but with t he diode in series to raise the threshold voltage... oh, and a resistor to limit the current. The STmicro L7812ABV has a tighter output voltage than many, 11.75 to 12.25 volts giving a threshold range around 12.95 to 13.5. The PNP will pull up to battery voltage when active, you can use an added N PN to turn that into a ground referenced output. Put the diode in the emit ter leg to lower the pull up voltage a bit.

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tect that if your car is one that works this way.

egulator, a PNP transistor and a diode. Connect the regulator normally wit h the diode base and emitter across the input (battery) to output, but with the diode in series to raise the threshold voltage... oh, and a resistor t o limit the current. The STmicro L7812ABV has a tighter output voltage tha n many, 11.75 to 12.25 volts giving a threshold range around 12.95 to 13.5. The PNP will pull up to battery voltage when active, you can use an added NPN to turn that into a ground referenced output. Put the diode in the em itter leg to lower the pull up voltage a bit.

Oh yeah, you'll need a resistor on the regulator output to carry the base c urrent since most linear regulators won't sink current on the output.

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Better to detect if the battery is being charged with a significant current.

Using say a Hall Effect ( non contact) device set up so that it is both level and polarity sensitive.

The it will need to latch and have some time out delay.

..... Phil

use 12V after the ignition switch, KL15 in DIN standard

Why?

Piggy back off the Fuel pump power. THe pump shuts off if the engine is not running after x seconds if the ignition is on.

Cheers

Take the power from the cigarette lighter feed.

Maybe the 12V after the 'acc' switch would be better? It's high when ignition is on, and can be user-invoked (with the key) with the motor not running, but is always off when you leave the auto.

Dashcams with internal battery, that trickle-charge, ought NOT to rely on generator operation for their function, else a car with bad fan belt becomes blind... but maybe suppressing the trickle-charge is appropriate then.

How easy is it to access that node?

the cigarette lighter might be on it, or on the radio connector

"Electronics be more reliable"? In a rather hot, almost still air environment? When cars are expected to run reliably in a desert? Give me a break.

Absolutely. Cars are far more reliable now than they ever have been, despite the one, with many zeros, times the number of transistors in them. It's called "design".

Tap the AUX power or the OBD-II port.

It'll certainly be in the fuse panel.

That's not reliable. Mine is always on. The AUX circuit is needed. Sometimes there is a switched AUX outlet (looks like a cigar lighter but it's not ;-) somewhere. Might also try a USB port.

If you have a car like mine, where the lighter socket is always on, maybe it would be better to move its connection to the switched power bus. Then you could turn it on in stealth mode without starting the engine.

I was looking for a fuse puller the other day and found they make fuse sock ets that plug in place of the fuse, with a socket for the fuse, plus taps f or external connections. Don't recall if that was on Alibaba or eBay.