Fade-in, fade-out LED circuit - for car interior lighting.

Restoring a Datsun 280zx, kind of a custom project, alot of custom body work, but now i've moved on to the interior.

would like to have LED's lining the headliner, and have them fade in when the door is opened, fade out when the door is closed. I think it should be simple. from what i gather i'm going to need:

555 monostable circuit

12v-5v DC power conversion

anyone have experience with this? looking to power about 15 LEDs... i know that the 555 can't support that many, so what to do?

Why are you converting 12V to 5V?
What fading circuit will you use? A 555 monostable just makes a delay before it turns on or turns off abruptly without fading.

this is why i came to the forums lol.

i don't really know. basically i need to know what circuits will be necessary to use 12v power from the vehicle to illuminate 15 leds, and fade them in/out when the doors are opened/closed

A new car uses a microprocessor and pulse-width-modulation to dim the interior lights.
Maybe you could do it with an opamp as an integrator driving a power transistor.

He can use the 555 as a PWM driver into a Power MOSFET into a Capacitor with the load acting as the R component in the RC Charge pump. Thus the duty cycle of the 555 governs the voltage seen by the load and controls the brightness via a potentiometer.

It will be awkward for doors opening and closing to activate a potentiometer. We need a voltage-controlled PWM circuit.
The duty-cycle of a 555 can be changed a little by changing the voltage of its pin #5 but it is not enough to use as a dimmer.

the fade out seemed to me like it could be done with a 555 monostable using a capacitor between the output and the LEDs, thus the door closes, a 5 second delay, then the 555 output goes low, and the cap discharges to dim the lights?

am i dreaming? and the fade in i'm having a hard time with.

A varient of the TI MSP430 would probaby be a good processor to use and do it via PWM.

But to entertain the other thoughts, you may not need linear dimming. e.g. The voltage across a capacitor might be able to contol a voltage to PWM converter. Velleman makes a voltage to PWM circuit board. So, I'm thinking...When the door opens apply full voltage to the PWM thingy. (Actually it has a slow start feature) and apply a buffered output of the voltage across the capacitor from a RC circuit. You probably will have to power the RC circuit from the always on circuit.

Watch quiesent currents.

When the door closes, I think there is a quiesent current issue.

What I'm thinking of is somthing that could charge a cap when the door is open and remove the charging source and connect a resistor in parallel when the door closes. Use the buffered voltage acroos the cap to drive the PWM thingy. I think quiesent currents will get in the way. e.g. the current drain on the battery when the car is off.

That gets you increasing brightness when you open the door.

Maybe just a simple RC on the gate of a mosfet will do the trick. The mosfet will dissipate a bit of heat during the ramp up/down, so if it gets hotter than you'd like, put in on a heatsink (insulated from the car body).

Midnightcarving
there are two options to do this:
1) analogue
this can be done in a few ways, using 555 or I would suggest some OA circuit, i.e. based on LM324 - this would be much better than 555.
here is a good example: http://www.solorb.com/elect/pwm/pwm2/
in the first schematic you need to replace the 3.9k 10k 3.9k regulator with RC circuit, that's all

pros: easier to complete once you have proper schematics, low cost, no need for special eqipment
cons: such a design can involve electrolitic capacitors, which tend to loose their characteristics over a long time period, especially in hot environment.

2) digital control circuit based on microcontroller
this is my preffered way nowdays , it is more precise, easily adjustable in firmware, and needs only a couple external components versus analogue option.
pros: better design, less components, smaller size, more reliable
cons: you need someone to help you with programming of MCU, this includes
a) writing a program
b) using some hardware to download that progam to MCU.
Overwise you need to learn it by yourself plus buy (or make) some basic programmer hardware, this may be not what you are looking for, if it is just a one-time project. If you want to learn something in this direction this project is a good way to start though.

Let me know if you need any help with 1 or 2a

p.s. Schematic for option 2 can be as easy as this: (add LM78L05 for MCU power)
**broken link removed**

dougy83
nice and clean schematics, will perfectly do the job. However, things that I don't like about it:
a) it is not linear
b) depending on the MOSFET used, the Vgs threshold may vary as well as Rdson linearity.

No relevant for this kind of project of course, just some thoughts...

zman, thank you so much. It is a one time project, but i'm finding myself thinking of more and more projects that will require programming. Time to learn it.

I'll be back with some questions for sure as i move through this.

You'll need a resistor inline with the switch to limit the 12V from switched to pwr doors. Otherwise the PIC input will go bye bye. It likes no more than Vdd.

Mosaic
agree!
but this was just a sketch. decoupling caps and other things are not shown.