Dome Light Dimmer

I found this schematic on another site. It is supposed to be for the dome light dimmer on the Audi TT. The light fades in when the door is opened, and fades out after being closed.

I did a little research and found that the IC can be had for $0.71

and the transistor sells for $0.39.


Sounds like it would be a cheap and fun little project for the car. I'd like to find some flat 2-conductor wire that I can run under carpet or between the spyder and the headliner surreptitously.

Any comments from the EEs in the crowd?


Ron Earp

Staff member
Hey Mark,

Since that is just a common 741 Op Amp you can get those in quad package design, that is, four 741s in one chip. You could then use each one to drive a different dimmer circuit - overhead done and footwells for example. Seems a lot of the newer cars have different lighting circuits that dim at different rates. Our Volvo dims the footwells quickly but the overhead at a slower rate, a neat effect.

GT40s got dome lights???

This is a variation of a standard voltage regulator circuit. In the voltage regulator, the car door switch is replaced by a resistor and the capacitor and resistors are replaced by a zener diode. Also, the sense line (pin 2 of the 741) is taken from the load. Basically, the capacitor and resistors (10K + VR1) provide a variable voltage reference, and the voltage across the lamp will track this. VR1 sets the charge rate of the capacitor. VR2's kind of a kludge...

Some things to consider:
The 741 is always drawing a small amount of power (probably not more than a couple of milliamps) from the battery. It's so small that I wouldn't worry too much about it draining the battery.

Automobile environments are subject to high temperatures and vibration. I would replace the variable resistors with fixed values once you find out where you want them set.
Also, the voltage rating on the capacitor is a little marginal for this environment - I would use at least a 25V rated cap. You typically have to derate the voltage rating on capacitors at elevated temperatures, so this provides an additional margin of safety. Also, use a high-temp capacitor (105 degree C). DO NOT use a tantalum capacitor here. Failure modes of tantalum capacitors are such that it could, if it failed, produce a direct short across your +12V supply. Electrolytic capacitors typically fail by leaking and messing up your circuit board, but they end up open circuit - that would just mean that the lights would come on full brightness immediately, which you can probably live with better than a blown fuse...

The 2N3055 is probably overkill, depending on how much current the lamps draw. It comes in a big honking TO3 case. You could probably replace it with another NPN transistor in a TO5 or TO220 case, as long as the transistor can handle the max supply voltage & current that the lamps require.

The prices you quoted for the parts look like quantity prices. I didn't look at the link you provided, but for small quantities of parts like this, try Mouser or DigiKey.
Gees! When you Eggheads get together it scares me. /ubbthreads/images/graemlins/confused.gif /ubbthreads/images/graemlins/grin.gif
Not to mention the fact that I am electrically and digitally challenged. /ubbthreads/images/graemlins/shocked.gif /ubbthreads/images/graemlins/crazy.gif /ubbthreads/images/graemlins/grin.gif /ubbthreads/images/graemlins/grin.gif