Comparator circuit.
 

here or homebrew ?
And it's a car problem. Easy solution is to fit a piggy back unit ,where the original circuit is fused through the main unit and the secondary supply is fed via a tap, and secondary fuse, to control a relay to control the supply from the non switched ciggie lighter. So that when the ignition is off, a supply is live.
But- getting a supply behind the dash means problems.
My alternative idea is to use a comparator on the vehicle supply rail. When engine is running, car volts = at least 13.5. Engine off, volts = circa 12.6 .
Great in theory, but can any of the wise men see any problems ?
Circuit would be a clamp on the non inverting input via a Zener and a prset on th non inverting input via a pot. Theory would be for output to be high when ignition off and low when engine charging battery.
Any ofthe wise men see any problems in this as it's a long time since I did any theory/practice on op amp comparators.

Re: Comparator circuit.
 

I am not 100% clear from your post exactly what you are trying to do, but I can point you at at a circuit which switches a circuit on when the battery volts go below a pre-set threshold https://www.eleccircuit.com/low-volt...itor-by-lm339/ .

I've used this for an alarm, set up so that when a 12V lead acid battery has its voltage fall below about 11V, the comparator switches on the buzzer. I guess the buzzer could be replaced by a relay?

B

Re: Comparator circuit.
 

This approach is likely to give trouble due to noise and spikes present in a vehicle supply. I would also suggest it's over complex. If you examine the vehicle fuse box and fuse listing, it should be possible to find a circuit appropriately fused and live as you want it.

Look on auction sites and find a replacement fuse link with a fly lead fitted to one end. This would do your job without hacking into the vehicle wiring and is easily reversed.

Leon.

Re: Comparator circuit.
 

Sorry, but car problems are off topic for the forums.

We do allow discussions about dwell meters etc. though,