Fade on, Fade off Interior lights

Theory of operation:

When 12vdc is applied the resistor slowly charges the capacitor.  As the cap slowly charges the voltage at the resistor/capacitor junction slowly rises.  The 741 op amp is used for its super high impedance (doesn't put a load on the circuit).  The op amp is configured in a voltage follower configeration, meaning whatever voltage goes in comes out the output.  In between the output of the op amp and the feedback there is a transistor circuit that is designed to simply amplify the low current signal coming from the op amp.  With the transistors in this configuration they will amplify the current of the applied analog voltage.  Be careful and note that the emitters are drawn in a strange way (I mixed it up the first time I used that circuit).  Because the transistors are inside the feedback loop they have virtually no effect on the operation of the op amp.  What you have coming out of the op amp is a voltage that slowly rises to near battery level (it will never actually reach battery level though, one downside to the op amp) with enough current to drive your light.

So, in the end, I used an RC circuit to drive a transistorized buffer that will dim the light on.  Now, interfacing that with the rest of the circuit is going to be another challange.

Oh, and I simulated this in Electronics Workbench, so it should work.

I didn't put part numbers on the transistors either, the top one is an NPN and the bottom one is a PNP.  Depending on the current requirements you could go with a 2n3904/2n3906 (if under 200mA).  Over 200mA you'll have to find bigger transistors.

Impressive, might actually inspire me to get the parts and try it out. Thanks

As far as mspaint I could use some colors but the diagrams would still look like stick figures, just in color. For circuits like above this is fine but I kinda like the "cartoony" looking ones for more basic stuff. I am very artistic, but can't draw that well. Perhaps something to do with being a lefty.

Hey guys, do you think this would work okay? I  I know it will need to be adjusted here and there, but for the most part it looks good to me. Basically, what it is supposed to do is take approximately 6 seconds to fully fade on. (Thanks KPierson for the diagram!) Then when the domelight wire ceases to  power the circuit, it will drain the 300,000 uf cap and the 220,000 uf cap, and will fade off for approximately 7 seconds. Which in turn is the fade on-fade off circuit I have been trying to figure out.

Now as far as trying to get it to stay on for a couple seconds, it will do that as well approx. 4 seconds. Which will be fine. I have a switch inside the truck to keep it on longer if I need to.

Looks good to me. There will also be a slight v-drop through the diodes barrier voltage (if I remember correct it's .7 for ge and  .3 for si) but without actually building/testing this circuit I don't have the abilty to predict the time length/brightness loss with any great accuracy.

The parts are not expensive so you might want to build it and post the results. I am curious myself.

Good luck