Dimming interior lights?

[redwagon]

I kind of miss the way the interior lights in my old car slowly dimmed when the doors shut. The Impreza's sudden snap into darkness is a little 'econobox' for my liking.
Is there some trick electronic timing/fading device needed to perform this miracle of the luxury car, or is a capacitor in parallel with one of the bulbs going to do it?
If it is, how big a cap?

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[redwagon]

Bump...

C'mon people, nice easy one for ya!

I just love lights that dim slowly when you shut the doors, call it a fetish if you have to

Hmm my friend's CivicSi (ProgSi) does this. I *love* it for some reason. I always keep that interior light completely shut off, but if it dimmed maybe I'd put it on.

[Impreza Rider]

Maybe it's one of those things that Subaru forgot to put in the car...like the trunklight and folding rear seats.

Jim

[North Ursalia]

all you'll need to do is add a small capacitor to the + side of the dome light: should cost about 75 cents and take about 10 mins to do. Mine stays on because of the aftermarket alarm....

[8Complex]

North Ursalia - Hey man if you know what it takes, please do share! I'd be able to do it if I had the specs on the cap. but I don't know too much about electronics.

imprezarider, and a light for the glove compartment! thats a pain in the arse to find stuff at night.

[Synthesis]

seems like subaru had a thing agains lights, they sure made sure they had the fog lights correct tho hehe

[yebokmj]

Almost all the foresters seem to have the auto dim light. I don't know if this helps but that means that is available through Subaru.
Joshua
MY98 RS
Subaru Salesman

[kelley nelson]

Bah, you should have gotten an Audi.

[JZ oo7]

gotta agree with Synthesis, seems like subaru put all the lights in the front, instead of handy places to put them (like the trunk)...but i'm happy where the front lights are! did anyone ever count how many light MYs have in the front? turn signal, corner, headlight, foglight, and side marker! that's like 5 lights going on whne you singal left or right! love it!


JZ

[Sunrise City Rider]

Any electrolytic capacitor should work. The cap will charge up to 12volts when the light is on and slowly, but not too slow, lose its charge and you will see it through the light. I never tried it but thats how it supposed to work.
Zee

[redwagon]

Wow, look what I stirred up

1. Audi service here is robbery, mate of mine owns an 80 and his (Audi) service center tried to charge him for a valve lash adjustment!

2. My LS wagon has a light in the cargo area, so there

3. Any suggestions how big a cap we need to fade off a little 5W bulb?

Cheers!

[Avenger]

Is there any electrician out there that can give me the correct formula for capacitor, resistance and time? Is it CxR=T? If this is correct what are the suffix for these values? micro, milli, mega.. etc Thanks

[wac]

SIGH... (shakes his head) You guys are funny.

The formula for exponential decay of an RC circuit is given by V(s)=Vo*exp(-t/(RC))
where:
V(s) = voltage at time s;
RC = time constant (rule of thumb: 4*RC = time to decay to almost zero);
Vo = initial voltage (12V in this case)

Using a 5W bulb, we get use R=V^2/P = 12^2/5W = 28.8 ohms. That means a total decay time of
4*RC = 4 seconds = 4*(1 second)
will require a capacitor of
C = (RC)/R = (1 second)/28.8 = 34,722 uF, or 34.7 mF.

Now that's one big honkin' capacitor! Audio buffs will appreciate what this represents. You can play around with the numbers, but you'll always end up with a very big capacitor. It gets worse; lamp filaments don't have a linear light output vs. voltage curve, so you'll have to compensate for that too, and it usually means having to use an even longer time constant.

Most prudent lighting engineers will instead use a digital or analog time delay circuit, and dim the lamp via a power transistor of some sort...

-WaC
Wayne
Geeks-R-Us

Edit: typo

[This message has been edited by wac (edited September 29, 2000).]

[Big Tee]

yebokmj is correct the Forester has the feature. Perhaps it can be borrowed for this application.

[redwagon]

Thanks wac for setting me/us straight.

Hmmm, guess we're not going the capacitor route then
We don't have the Forester here. I'll check with my dealer if the '00 or '01 Legacy got this feature.
Have any ideas where the device might be fitted? How about in other cars, BMWs, Audis, Chryslers..? Where they usually hide these circuits?

[Impreza Rider]

Josh-

Think you can look up the parts for us? I had this feature on my 94 Galant, and I loved it...

Jim

[Rudypoo]

Just go to the library and look up something like an "Encyclopedia of Electronic Circuits" or something like that. They usually have automotive circuits in there and this dimmer is one of them.

[Bassem]

Check this website out:
http://www.velleman.be/kits/k5002.htm

I think you can get velleman kits thru radioshack or one of those electronic mailorder thingymajigs

[redwagon]

Hmmm, found this design, pretty complex but it does everthing short of open the doors for you.
http://www.webelectricmagazine.com/00/2/dome.htm

I could probably breadboard this up, but how the hell do you get the program into the thing though?
Sure would be a lot easier to source one from another car!

[wac]

Hey red,

That's a great find! Instead of the microprocessor version, try the 555-timer version instead: http://www.webelectricmagazine.com/99/1/dome.htm
You can find all the parts at Radio Shack or any decent electronics supply store.

If there's sufficient interest, I can even make the PC boards for what it'll cost me in materials (and shipping). Email me (see profile) if you're interested.

-WaC
Wayne

[redwagon]

It looks to me like the 555 version is just a timer...20 second delay, then lights out. The sudden snap off (like being inside a fridge) is what I'm trying to do away with!

[jackwei]

you're in Taiwan aren't you? :P
just go to ¥úµØ°Ó³õ and ask around!

[redwagon]

WaC, if you want to make the full-on device, count me in.

Jack, ¦b¥x¤¤,¬O¹q¤lµó°Õ

(Sorry to all of you guys without Chinese systems...)