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One thing I really like to have in a car is a warning buzzer than tells me when the headlights are on when I open the door, so that I don't leave the lights on and drain the battery.
My vintage Beetle never had such a circuit. It did have a buzzer that would buzz if you left the key in and opened the door. And it also turned on the dome light when either door was opened, independent of anything else. However, the switch to make both of those things work independently is a switch you can't get any more. It's a double-pole switch that switch the circuits independelty, and neither of them to ground. The only switches you can get for the doors are single-pole grounding switches. So the circuit to do all of this with a single switch per door gets a a bit more complicated.
I've designed a circuit to do the job. I want to have the buzzer go off if I open the door and any of the following is true:
Here's the circuit; two relays (they can be small ones) and four
diodes. Now I just need to get all the connectors organized and the
wires run, and the componnets wired up.
The only problem with this is I haven't been able to figure out to
wire the relays so that if either the door is opened or the key is in,
there isn't current being drained from the battery. Ideally it would
only draw when both were true. But for the moment, I'll have to put
up with a bit of current draw when the door is open, even if the dome
light is turned all the way off. That's probably not that big of a
deal.