Most automotive vehicles typically have their interior lights fade out 30 seconds or so after the doors are closed.
The way I understand it, closing the door will usually depress a momentary on/off switch, opening the circuit, isolating the lights from the battery.
Even with that main circuit broken, the lights manage to stay lit for a short period of time before dimming and fading out completely.
I always assumed this was attributable to the use of an inline capacitor but have noticed some aftermarket solutions seem slightly more complex than that. (Images below).

What's actually involved or required in these circuits? ie.
What function/purpose do the additional components provide/serve?
Are they necessary?

1. I understand that an increasing amount of newer vehicles are probably providing this functionality via ECU integration. If your answer is about this; it's probably unrelated to the actual question.
2. This question is more concerned with the electronics involved and is not necessarily exclusive to automotive applications, despite the example provided.

1a 2a 3a 1b 2b 3b

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    \$\begingroup\$ The capacitor needed to create such delay would be huge, also the discharge curve would make it dark before most of the charge was gone, so its also quite inefficient. \$\endgroup\$
    – Wesley Lee
    Commented Nov 10, 2015 at 4:56

1 Answer 1


The device you show is connected directly to the battery (well. through a fuse) so it always has power. The door switch that normally powers the light is now just an input to to this circuit. It probably has a small microcontroller which turns on the light when it sees the door switch close (door opens) and does a delayed fadeout when it sees the door switch open.

As @WesleyLee noted in his comment, a capacitor that could keep a 10W light lit for 30 seconds would be prohibitively large and expensive.

  • \$\begingroup\$ Is it just me though, or does the first image appear to be only a 220uF capacitor, a diode and a variable resistor? \$\endgroup\$
    – voices
    Commented Nov 10, 2015 at 6:38
  • \$\begingroup\$ @tjt263 Look carefully, it seems that the switch transistor is mounted underneath the board. \$\endgroup\$
    – R Drast
    Commented Nov 10, 2015 at 12:29

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