Can I get some help to make an automatic light-dimmer 1

[allax]

Hi.
I'm trying to make a dimmer for the roof-light in my car, so when I open the door the light fades on, and when I close the door the light fades off (fading takes about 4 seconds).

How can I do this easy?

This is what I have imagined:

+ ------|
R
|
12V --R--|
| |
C Light
| |
- --------------

I hope the sketch is readable. A resistor and a capasitor in serial, attached to 12V. A resistor and the light in serial, attached in parallell to the capasitor.

So when 12V is connected, the capasitor makes the voltage to the light fade up, and when the 12V is disconnected, the capasitor discharges slowly through the light.

Is this a possible solution? Which values should I use on the capasitor and the resistors? If I can use resistors with low resistance, I could use 12V light bulbs.

I appreciate all help and hints. Thank you.

 

[blcpro]

A circuit like that might work, but the values of the resistors and capacitors would need to be large to get the desired fading effect. A consequence of the large values would be a loss of overall brightness in the bulb. Another alternative might be to use a resistor-capacitor network to bias a transistor used to turn on the bulb. This way the only limiting factor in bulb brightness is the transistor. Much smaller power-rated resistors and capacitor could be used. Be careful with the transistor selection... make sure the transistor can handle the bulb power without getting too hot!

 

[peebee]

If you want to do this using all passive components, I'd suspect you'd want to put an inductor in series with the light -- no other resistors or capacitors. This will allow full output. The inductor could still be pretty big, though -- you might do better to go to an active (transistorized) circuit as mentioned by blcpro.

Regardless of the circuit topology:

To select the value of the inductor or capacitor, realize that the light is a resistor in an RC or RL circuit. The value of the resistance of the light in ohms is R=12*12/W where W is the wattage of the light. If you don't know the wattage, then either measure the resistance of the light with an ohmmeter, or measure the current I, then the resistance is equal to R=12/I. In both cases, the 12's come from the 12VDC source -- if you're doing a current measurement, you might want to check the voltage too, and substitute the voltage measured (perhaps 13VDC) instead of 12.

Once you have the resistance of the light, it's easy to select values for inductors or capacitors. Do a Google search on "RC time constant" or "LC time constant" for LOTS of information on this subject.

Here's another suggestion: instead of trying to build this thing, just stop by your local auto-parts store and see if they have the dimmer part for some other car that already has dimming, if so just buy it and slap it in.

 

[piedade]

Use an NE555 you'll find in the web plenty of circuit for that purpose

 

[skills]

Use an Atmel AVR and write a small program to do this. This
will be much easier than other approaches and give you more control over dimming curve!

 

[47gandalf]

Or get the module from a junked BMW. They have this function on the dome light and the light on the mirror behind the sun visors.

 

[Paulcet]

Gandalf,

A similar function was on an old Honda Prelude I used to have ('85). It came on instantly, but faded off over a period of about 5 sec.

 

[melone]

The new VW's do it as well...