Fading Dome Light

97SL1
One bit of class I miss in my saturn is a dome light which fades off
when I close the door. Is there a simple way to implement this?

I went through the calculations of a basic RC circuit, and to add a
capacitor to the system would require a charging current which would
pop a fuse when I open the door... long story short: *theoretically* a
basic parallel capacitor won't work...

The second idea I have is using an small RC model curcuit to set the
discharge time, and use a voltage buffer circuit ot feed the bulb...
however I believe there should be an easier way.

Any ideas????? (preferrably pre-built )

Thanks!

 

Try a resistor in series with the capacitor to slow the in-rush.

 

If you have a LARGE capacitor, two diodes and a resistor, this approach will
work. You charge the capacitor through a resistor and a diode. The resistor
can be sized so that it will limit the current into the capacitor so that
the fuse doesn't blow. You also supply the current to the bulb through
another diode, and have a diode from the capacitor to the bulb.

However, there is an implementation problem with this approach - it requires
a capacitor on the order of a Farad. Those are large (about a tall-boy beer
can) and relatively expensive.

A simple two transistor, one capacitor and a resistor or two will do the
trick. It's a bit beyond the scope of a newsgroup post to describe though...

 

Will a 2 transistor amp handle the power requirements? My practical
experience with transistors has been limited...

Also, does anyone know of a module (from a junkyard or so) that I
could just 'plug and play' type thing? I think most of these things
are pretty integrated, however just curious

Thanks again for the input!

(And yes, a 1F cap is out of the question!!! )

 

Actually a 1F cap is about $2 on digikey.com if you can deal with a 2V or 5V
rating. The supercaps are getting cheaper every day.

 

1) You'd need about a 15 Volt rating if you wanted to use the cap to
directly feed the bulb.
2) The small super-caps have a very high internal resistance. They're more
oriented towards supplying the low currents over a long period of time that
are used for applications as memory backup. The capacitors that have large
values (on the order of a Farad), sufficient voltage (on the order of 15
Volts) and sufficiently small internal resistance are still about the size
of a tall-boy and cost about $100.

3) PS - Yes, a two transistor circuit would be able to handle the power.
And, since the output transistor would only be dissipating while the light
is being dimmed (not when it's full on or full off) you probably wouldn't
need a heat sink. Of course, if you sit there turning your light on and off
repeatedly, you could overheat the transistor this way...

 

I found solutions!

I found a homebrew solution at:
http://www.mashhadkit.com/circuit/Dome light dimmer for Cars.htm

I also found a prefab solution at:
http://www.webelectricproducts.com/products.htm


Thanks for your help!
Cheers.

 

The one with the OP-AMP is a little over-engineered - a simple two
transistor circuit will do the trick.

But, 741s are cheap and commonly available, so I see no reason that way
shouldn't work just fine.

 

That's the one that would pop the fuse, due to the large cap needed.

Simplest solution is an R-C circuit driving an emitter-follower,
possibly in two stages.