[Oldcodger]

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.