How can i make the LED fade out slowly?

hi all, i only know a little of electric things.... so
would someone pls teach me how can i make the LED fade out slowly when i turn it off???

use capacitors????
how can i choose the right capacitors to fit the led?
are there any formula for it (like the one to calculate the correct resistors to fit the LED)??

here is the LED spec tat i want to make it to fade out slowly..
DC forward current 20mA
Peak Pulse forward current 150mA
DC reverse voltage 5v
power dissipation 80mW

the power i am using will be 12v... i am using it in the car

thx a lot for help

Alot depend how fast you want it to dim out. A super cap from an old PC motherboard will work for a LONG fade out. .22Farad (220,000,000uF) rated at 5v right across the LED should take 3-5mins to totally fade out.
Jeff

Will using such a large cap drain much of your power and affect output? /ubbthreads/images/graemlins/icon3.gif /ubbthreads/images/graemlins/icon3.gif /ubbthreads/images/graemlins/icon3.gif /ubbthreads/images/graemlins/thinking.gif /ubbthreads/images/graemlins/thinking.gif /ubbthreads/images/graemlins/thinking.gif /ubbthreads/images/graemlins/huh2.gif /ubbthreads/images/graemlins/huh2.gif /ubbthreads/images/graemlins/stupid.gif /ubbthreads/images/graemlins/rolleye11.gif

from what I have seen, these caps take a little bit to fully charge, so they wount suck the power going to the LED while it is charging, but I will check this. If the LED is current limited to 20mA and the cap is across the LED, the total draw shouldent exceed 20mA or so.

Hmm..

So basically you just wire up the batteries, switch, resistor and LED the normal way and place the cap across the LED? No special charging requirements?

Sorry, I'm a dumbass when it comes to electronics. I solder wires, but that's about it /ubbthreads/images/graemlins/blush.gif

thats about it. Since the cap is across the LED, it will only charge up to the forward voltage of the LED. A larger cap will cause it to stay lit longer, but a .22F cap (or larger) will take longer to charge up, it has a long charge slope.
Jeff

sounds like a cool feature. A ramp-up option might be possible with the same hardware.

Forget the above, and try the following.

Get an N-Channel FET and use that for the switching.

You will switch on the LED by driving the gate voltage high.

So...

Connect a capacitor from the negative ground on the car to the gate. Connect a resistor across this capacitor. The switch off time will be roughly 'Capacitance in Farads' x 'Resistance in Ohms) Seconds.

You can then either use your switch to connect the gate of the FET to the +12 volts for an 'instant on' function, or you can put another resitor in series to get a 'fade on' effect. again using RxC to calculate the time delay.

Don't forget you will still need a series resistor to limit the LED current.

thx for da info guys...

To WildRice :
so connect the LED in normal way then place a cap to + n - of the LED??? i only need the led to fade out 3-5 seconds... wat cap should i need?? would you pls find me the correct one in this page?
http://www.dse.com.au/cgi-bin/dse.filereader?427128960914bdb4273fc0a87f9c0717+EN/catalogs/CTG0000602
i am so confuse to find the right one....


To UK Owl :
so your setup is

12v --------> N-Channel MOSFET--->resistor--->LED
Ground->cap->
and place a resistor at the both side of the cap?

(sorry about this diagram...lolzz /ubbthreads/images/graemlins/smile.gif )
are there any different N-Channel MOSFET?(different spec?)
or is only have one type? if there are many different type... would you pls tell me wat model No. i have to get.

again.. 9999999thx for da help
TOM

I will try some stuff tonight. if you only need a few seconds fadeout, a super large cap isn't needed. I try some different caps with LEDs tonight.
Jeff

thx wildRice

Neat.. I might give that a shot.

Any idea of how well this cap idea would work with a Luxeon 1W at ~300mA? It sounds like a cheap and easy way to go about doing things especially when I am throwing away computer motherboards (that I can salvage caps from).

Well, it all depends what you are looking for. 4700uF will work, here is how. When power is removed the brightness dropps to about 60%, fades to 20% in about 4secs, then takes about another 2- secs or so to completely go out. the .22F dropped to about 60% also, but took over 2mins to fade out. If a smooth fade from 100% is needed, the N-FET is probably the way to go.
Jeff

Connect a diode 1N4007 between a 100 uF - 16 V electrolytic cap and the battery. Place a 470 Ohm 0.5 Watt resistor between the + of the capacitor and the LED.
When you remove the 12 V power, you have a nice 5 seconds fade out without killing the LED, or other similar annoyances.

Anthony

Cool, I've wanted to do this with a 3W Cyan led for a while. I think it would make it seem like a high powered phaser or something. Turn it off and see it fade out in 5 secs like it's powering down. It would kill the ability to rapidly signal or flash people though. Which I've used it a lot for. But the fade out would be so cool I'd probably play with it more doing that!!

If anyone does get this workign well with 5mm or Luxeons please post the part specs!!! Or if you have access to a bunch of suitible capacitors perhaps offer them for sale? /ubbthreads/images/graemlins/popcorn.gif

Hmm, Also it would make people think aliens are visiting them!! heheh I think 3secs fade to black would be best...

learner - here is the circuit I was on about...



You can use any N-Channel FET and it will not matter much with such a simple circuit. If you want to use this with a Luxeon or other high power LED then remember to get a high power FET. The value for R1 given will give about 25 mA through a white LED from a 12 volt car battery.

Remember you can change the on/off delay by changing the value of R2 and R3.

How it works....

Think of a FET as a variable resistor that changes from infinity ohms with 0 volts at the gate, and goes all the way down to 0 ohms when the gate voltage rises to about 3 volts. (Not quite true but a good enough description for a beginner to understand how to use FET's)

When you switch on the capacitor voltage will rise at a rate determined by R2. When you switch off the voltage will decay at a rate determined by R3.

To change speed of operation think - lower resistor value = faster operation.

I have not built this circuit as such, but have made ones operating on the same principle. The only problem will be a delay when 'off' is selected due to the voltage at the gate being 12 volts needing to decay to about 3 volts before any reduction in light level is seen; this could be addressed by adding a zener diode (in parallel with the capacitor) to limit the voltage at the gate of the FET, but this will make the circuit a little more complex.

p.s.

If you are in the UK I can post you one or two ST72A N-Channel FET's, however I have no specs for them as they were a cheapie bought on e-bay (and can not find a data sheet for them) but I have used them before and know they will do for this circuit.