trying to add some lighting in the foot wells and under the seats in my car so that when someone drops something down there in the dark, it's not something that (at least not every time) requires pulling over and grabbing a flashlight to see. LED strips are cheap as chips, so my thought is I would just tap a 12V power supply, but then thought about it more, and technically I would want these on in 2 cases:

1) If the headlights are on (would only have power while the car was running; essentially: if the dashboard is backlit, the foot wells should be lit as well)

2) If a the dome lighting that corresponds to whatever area is on (in my car, there's a light in front, and another in the back, so if you open the front door, I'd want the front foot wells lit; open a back door, light the under-seat lights)

I am able to put stuff together once provided with an example, and assume I need some type of switching device to make it so I'm not turning on dashboard lights when I open the door / blowing anything up, but, as of yet, haven't figured out what to do there. Any input would be helpful.


2 Answers 2


With limited information about the components you want to use, it seems like a "diode-OR" circuit (Wiki Link) could we a quick and dirty fix. If either the dashboard or the front dome lighting is on, the LED strip will be powered (minus the voltage drop across the diode). However, the dashboard will not power the front dome (or vice versa).

Basically, something like this, which you could obviously replicate for the back foot wells:


simulate this circuit – Schematic created using CircuitLab

Also, this assumes the LED strip is rated for 12 V and has internal current limiting features; or else we'd want to add a current limiting resistor in series.

  • \$\begingroup\$ This sounds like what I was looking for; a friend of mine has a similar set of LED lights for under his dash and gave me the instructions, but his are on all the time when his car is running, or when he opens the door. The kit he got was a cheap one with sparse (and barely English) instructions but in there it seemed like they wanted you to connect the ground to the ground of the dome lights if you wanted it to come on when the car was off and the door was open. I'll start sorting out what wires are pushing power to the different domes and see if I can get it working. Thanks for your help! \$\endgroup\$
    – i_djon
    Oct 25, 2016 at 18:48
  • 1
    \$\begingroup\$ You'll want bigger diodes than a 1N4148 for this, but yes :) a pair of 1N4002s will get the job done in a pinch, while some 30 or 40V/1-3A Schottky's will be a better choice. \$\endgroup\$ Oct 25, 2016 at 22:22
  • \$\begingroup\$ @ThreePhaseEel good call - I should have removed the model # from the diodes. Agreed, they'd be too small. \$\endgroup\$
    – Jim
    Oct 26, 2016 at 1:20

The reason they wanted the ground of the LEDs tied to the ground of the dome light is that most cars have 12 volts wired directly to the bulbs, and the cheap and simple door switch closes to ground (read chassis ground). I say most because my 2012 Mustang interrior lighting comes on gradually like a dimmer. Im guessing theres a simple PWM circuit in the "body control" computer that handles this function.... got to remember, a lot of old mechanics knows nothing about electronics.


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