Fade on, Fade off Interior lights

This should be a relatively simple circuit to design/build. I used to do stuff like this back in school but forgot just about everything..lol

I will look through my old notes so if nobody else can post a suggestion know that I am on the case.

I am pretty sure this would work. I think...... What do you guys think?

Now another thing, is I would like it to stay on for approximately 20-30 secs when I get out.

I will have to look into it, not sure if that circuit will work or not, might need a resistor, and also will depend on current draw ( the sum of how many lights for the interior, or if only for one light the current draw of that) to determine length of time for fade. If there is a wattage rating for the bulb(s) let me know and I can use ohms law to determine current. Most likely only 3-5a each light.

As far as the lights staying on when you get out, most  newer vehicles have a delay as standard equipment. What kind of vehicle do you have?  You can use an off the shelf timer circuit and relay to do the delay but I need more info about your vehicle. IE: Is it a positive or negative switch for interior lights?

What you can also do is buy a few caps of various mfd's and use a 12v light bulb similar to the one in your vehicle and a 12v power supply or battery. Set up the circuit and see what happens when you turn the light on and off. Cheap and easy way to find out if it works.

Personally, I think you'd either need to go with a transistor controlled by an RC circuit, or use PWM to control the light.. both ways have good and bad points.

The circuit as drawn won't work the way you want it to.. you're on the right track with thinking about using caps and resistors, but you don't have a resistor in the circuit.  Do a search on RC circuits.. Resistance Capacitance. PWM is Pulse Width Modulated.

http://webphysics.davidson.edu/physlet_resources/bu_semester2/c11_RC.html

http://www.rwc.uc.edu/koehler/biophys/4g.html

Here's a page with more application specific information..

http://www.discovercircuits.com/L/lite-dimmer.htm

Personally, if you're interested in electronics then this one is _easy_.. an RC circuit driving a transistor is a pretty basic circuit - but you'll likely want to play with it on the bench.. Read through the links I provided.. if you don't have any interests in electronics, then find someone to buy a module from..<grin>

Any questions.. feel free to ask.

Jim

Good links Mad Scientists, your name suits you. You seem to always know the whacky stuff.. Those links should help with the "fade" circuit. I knew there was a resistor involved but it's been a while since I have done this stuff. And after seeing those formulas again I really don't miss it..lol

As far as the 20-30 second shut down delay I still think a timer/relay is the way to go.( adj from 0 to 60 or 90 seconds I think) DEI has one and it's inexpensive but may be others around too. Just to simplify things. Using a diode to prevent back feed we can trigger the timer with the appropriate polarity when the doors are closed and have it send the approriate polarity for the dome light to remain on and adjust for the time desired before it "fades" out with the RC circuit.

Keep us posted.

Thanks everyone, this is helping quite a bit.

"What kind of vehicle do you have?".................1986 Ford Ranger

"Is it a positive or negative switch for interior lights?"....................Positive (I never checked it) But I know how to reverse polarity with a relay, so I'm capable of making it work for this circuit if need be.

"Most likely only 3-5a each light"..................I am not sure, can I determine it with a DMM?

"The circuit as drawn won't work the way you want it to.. you're on the right track with thinking about using caps and resistors, but you don't have a resistor in the circuit."..........................Yeah, I know this circuit is not going to stay on for any period of time, it is just meant to fade on, then fade off. Would I need a resistor in there? Which of Ohm's or Kirchoff's laws do I need to determine the size of the resistor?

Sorry, I have so many questions, but I'd rather hear the right answer! I also have the Basic Installers Studyguide, I have read it twice but I am still unsure how to apply some of the laws to this application. If anyone can actually post one full equasion with a brief description, I could go by the example to determine my own. It would be greatly appreciated. BTW I liked those sites Jim, now I have them added to my favorites list.

Again, thank you for any, and all info provided!

"Which of Ohm's or Kirchoff's laws do I need to determine the size of the resistor?"

Sorry, let me correct myself..... 

Which of Ohm's law is needed to specify the correct resistor?

Which laws do I use to determine the amount of time that the light stays on? Kirchoff's?

I'm guessing that the diagram posted above would allow the light to fade off (the filament would be the bleed resistor that would allow the stored current of the cap to discharge).  However, it won't fade on, and it won't delay at all.

You can get time delay relays that would integrate with what is posted above to give you a delay, then a fade out.  I personally don't think the fade on is that necesarry.  I like instant light! 

I found out that the only bulb that it will control's specs are:

volts: 12

amps: 0.833

watts: 10

"However, it won't fade on, and it won't delay at all."

It wont? Even though the capacitor will be taking in some of the charge?