Inspired by
this circuit, I decided to build a soft off circuit for my map lights. The idea is to allow the lights to stay on for a few seconds after I've entered the car.
The basic effect is illustrated in
this movie. I increased the delay a bit since this video was taken.
Update See the circuit in action on my map lights:
Movie
The version I've constructed is only good for LED lighting applications; I get away with using a small signal NPN transitor for Q2 because my LED map lights draw less then 70mA. Incandescent bulbs could draw upwards of an amp, especially when cold. Replacing Q2 with a MOSFET allows this circuit to be used by those with incandescent lighting as well (see above link). If you build the incandescent version, installation is exactly the same.
Here is the circuit:
It's very simple, as you can see, and component selection is very flexible. All component values were chosen based on what I had on hand. anything reasonably close to the quoted values should work just fine. Delay lasts about 5 seconds with a 100uf C1 and 500R R5. Higher values for C1 and R5 will increase the delay time. Anyone purchasing components for this circuit does not need a 100uf cap, I used it because I had it on hand. 20-30uf should do the trick with a larger R5.
I constructed the circuit on a breadboard prior to soldering, this allows for experimentation and fine tuning of the delay time. If you would like more control over the dimming time a variable resistor can be substituted for R5.
After soldering and testing the circuit, it was time to install it.
I completely removed the map light assembly from the car for the install. Just grip the sides and pull hard; It takes quite a bit of force. Unplug the wire harness, and proceed to the workbench.
Remove the black plastic piece from the gray housing by bending the plastic out at each of the tabs. A small spring and ball bearing will pop out of the "Door-off" switch as you pry out the black plastic. De-solder two wires on one side to allow the board to flip over.
Now we can access the light wiring without damaging the original wiring harness. With a pair of pliers I clipped the trace marked "door switch/trigger". This allows us to wire in our delay circuit. There is no need to cut the other traces, cut
only the door switch trace. With this method the delay only functions when the door is opened, not when the map lights are turned on with the switch.
The +12V wire from our circuit should be soldered to the +12V trace and the ground wire should be soldered to the ground trace. The 'door switch' wire and 'trigger' wire are soldered to the cut trace; solder the 'door switch' wire on the side going out to the wiring harness (upper left in the image) and the trigger wire on the map light side of the cut.
Go crazy with electrical tape, and reinstall the black plastic into the gray housing, remembering to re-solder the switch wires and re-install the ball bearing and spring in the "door-off" switch.
Now return to your vehicle, plug the map lights back in and try it out. If you haven't screwed something up your lights will now have a delay and a pleasant dimming effect before turning off completely.
Hope this is useful to someone. Zoom-Zoom :-D