Fading LED light?

I was just wondering if it was possible to make a LED light fade on and off? TIA

You mean like the one on the front of the car player?

It's done with pulse width modulation I think.

Well not fade on and off continously, just fade on when it gets power and fade off when power is taken away.

Do you mean an LED or the empeg's LED. Both answers are probably yes, but the more precise answers are quite different.

Assuming you mean the prior, an LED can appear dimmer by pulsing electricity through it so that it is not lit 100% of the time. That is, the LED will always glow at the same brightness, but, using persistence of vision, our eyes can be tricked into thinking it's dimmer. So you could then increase the percentage of lit vs. unlit time and make it fade on. But this is probably not simple circuitry (not that I'm an EE).

Yah I meant an LED. I installed one in my car so that at night it glows a dim red. I just want it to fade on when i turn my key and to fade off when power is taken away, which is by opening my door.

What is the LED for? Just a random LED in your car?

Kinda, its a red ambience light basically. It looks really cool in my car because all of my ineterior lights are red. Its in the newer Audis, and I wanted it in mine.

Or alternatively just change the voltage. If this is for an application unconnected with empegs and the LED is to be used as an indicator, we can do simple things with just a capacitor which will smoothly dim the LED when power is removed.

Ages ago I made an earring which worked under this kind of principle - multicolour LED through the ear with circuits connected to the Red and Green leads which varied the voltage over time (2 sine wave generators at slightly different frequencies) to give some cool/odd effects.

Hmmm. It was my understanding that LEDs always glowed with the same intensity, regardless of voltage (obviously, there's a threshold at which the intensity becomes nil, but I think we can assume that, anyway).

Maybe you were dealing with ``special'' LEDs? Or maybe I'm an idiot?

LED glow intensity definately vary depending on voltage - but the voltage difference between dim and intense is pretty small (you might want to look up the discussion on the benefits of individual resistors compared to a single resistor to power the 4 LEDs in the buttons for the illuminted button hack). The difference in current, relatively, through the LED is much larger - the UI diagram of a LED is a typical diode "knee" curve where the current stays apprx 0 until the threshold voltage is achieved, then current increases strongly for small increases in voltage.

Intensity is related to current, since it's the individual electrons falling through the band gap that each emit a photon. Different colored LEDs typically have different forward voltages (the threshold voltage where the start to emit light) related to the energy (in eV's) of the emitted photon. The photons energy is proportionate to it wavelength (color).

/Michael

Yeah, fading LEDs are usually PWM (pulse width modulation)....you might want to check you the Fan discussions in the technical forum - the same fan controller chips we're discussing there could easily be used to do LED fading. Replace the thermistor with a pot, and modify the output stage for the LEDs forward voltage...

>Maybe you were dealing with ``special'' LEDs? Or maybe I'm an idiot? <

Neither - I just experimented with a battery and pot and found out which range gave me "almost off" to "almost blown up" and biased my sine waves off the centre of that range.

PWM could well give you more precision, but for under £2 my circuit did all I needed.