Fading Dome Light

Discussion in 'General Motoring' started by Nathan, Dec 24, 2003.

  1. Nathan

    Nathan Guest

    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 :) )

    Nathan, Dec 24, 2003
  2. Nathan

    Skid Guest

    Try a resistor in series with the capacitor to slow the in-rush.
    Skid, Dec 24, 2003
  3. Nathan

    Kirk Kohnen Guest

    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...
    Kirk Kohnen, Dec 24, 2003
  4. Nathan

    Nathan Guest

    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!!! :) )
    Nathan, Dec 26, 2003
  5. 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.
    Igor Orlovich, Dec 26, 2003
  6. Nathan

    Kirk Kohnen Guest

    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...
    Kirk Kohnen, Dec 26, 2003
  7. Nathan

    Nathan Guest

    Nathan, Dec 27, 2003
  8. Nathan

    Kirk Kohnen Guest

    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.
    Kirk Kohnen, Dec 28, 2003
  9. Nathan

    Dan Hicks Guest

    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.
    Dan Hicks, Dec 28, 2003
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