How to make my LEDs fade out?

[theMACHINE]

I'm building a Ruby-ish thingy (part noisy cricet, part my own secret real cool stuff), and It has 2 LEDs. Anyway.. I want those LEDs to fade out whenever I turn the switch off to give a nicer feel to the whole thing (and to make it seem fancier  ). The way to do this would probably be with a couple of capacitors, right?

But do I wire it in parrallel right before the resistor, or what? I have no proper capacitors to play with at home right now, so I'll have to go fetch some tomorrow. Anyway, what do you think would be a good capacitance value for a nice 1-2 second fade?

Jag fes precis en sån jävla vidrig fjärt, grabbar.

[petemoore]

  disregarding the current limiting resistor for 9V led operation.
  You want 9v to drop to 0v more slowly, adding large capacitence will supply voltage after the power is lifted, bigger = V drops more slowly.
  I believe I have this feature as a consequence of using many PS filtering caps at the circuits. Because they are all parallel, [they all go to the same ground and V+ line] they add.

[MikeH]

Taking a bit, fat electrolytic (I'm talking 1000uf or so) across the power supply, be it battery or DC jack, will make the whole thing fade out when you turn it off.  Experiment with values to get the right "feel" and duration.

[theMACHINE]

Taking a bit, fat electrolytic (I'm talking 1000uf or so) across the power supply, be it battery or DC jack, will make the whole thing fade out when you turn it off.  Experiment with values to get the right "feel" and duration.

Yeah but then the other (grit) LED won't fade when switched.

[GibsonGM]

The cap trick would work when you turn the power switch off (as in unplugging at the jack, if you use a ground switching jack)....but since the effect is really always on, doesn't it sound like a B F'ng cap to ground is needed after the limiting resistor, so that the LED will fade out when stomp-switched?  A PS cap would only do this when the plug is pulled.  Of course it can't be in series, so would have to go to ground after the resistor....I think, LOL.  Maybe before it.

?

[GibsonGM]

seems like you'd have to do it with each LED, at its switch.  Select the cap for the 'delay' in discharge.  R in ohms X Capacitance in F = seconds. 

[theMACHINE]

I found a 2200uF cap laying around and tried my idea... It takes like over a minute and a half for the LED to completely fade 

But now I know it works.

Well this gave me an idea!
Would'nt it be possible to charge up a really big electrolytic cap via the 9vDC and have a switch to use it as a built in rechargable battery drving the complete amp?

[Speeddemon]

I found a 2200uF cap laying around and tried my idea... It takes like over a minute and a half for the LED to completely fade 

But now I know it works.

Well this gave me an idea!
Would'nt it be possible to charge up a really big electrolytic cap via the 9vDC and have a switch to use it as a built in rechargable battery drving the complete amp?

But then you'd have to use a 10000uF cap for a 7,5 minute song, before having to reload, right? 

Damn, 10000uF, does that even fit in a 1590B enclosure?  I once needed a 15uF cap (rare value ofcourse) and they gave me a 1000V version, so the thing was as big as 1 AA battery. And usually such huge capacitances (10k uF) have high voltage ratings as well, right?

[theMACHINE]

I found a 2200uF cap laying around and tried my idea... It takes like over a minute and a half for the LED to completely fade 

But now I know it works.

Well this gave me an idea!
Would'nt it be possible to charge up a really big electrolytic cap via the 9vDC and have a switch to use it as a built in rechargable battery drving the complete amp?

But then you'd have to use a 10000uF cap for a 7,5 minute song, before having to reload, right? 

Damn, 10000uF, does that even fit in a 1590B enclosure?  I once needed a 15uF cap (rare value ofcourse) and they gave me a 1000V version, so the thing was as big as 1 AA battery. And usually such huge capacitances (10k uF) have high voltage ratings as well, right?

Well you can always drill a hole in the enclosure and do the "tube bragging thing" but with a cap.
But this was probably a bad idea anyway, so forget about it.

[theMACHINE]

To get them to fade in is a bit tougher huh?

[remmelt]

Use a slow draining supercap? I don't know if you can get them for 9V though.

[axg20202]

You could probably do it with an LED microcontroller instead. Google for them.

Andy.

[John Blund]

Couldn't the led be hooked up like the millennium bypass v1, supposed to fade out over 1/2 second or so? 
or was that fade in?

[Paul Perry (Frostwave)]

Have a separate electro for the led & its resistor.
Feed this cap from the power rail via a diode.
That way, when you turn off, the cap can only discharge via the diode, and it will last much longer than if the whole circuit was across it.

[theMACHINE]

Have a separate electro for the led & its resistor.
Feed this cap from the power rail via a diode.
That way, when you turn off, the cap can only discharge via the diode, and it will last much longer than if the whole circuit was across it.

you mean like this?



Thats what I planned on doing, but thanks anyway.

[Elektrojänis]

And usually such huge capacitances (10k uF)

10k µF = 10000µF = 10mF = 0.01F

The unit is really Farad. It is ust mostly too large to use so we normally speak of microfarads (µF or uF). 1µF = 0.000001F

have high voltage ratings as well, right?

Not necessarily... In car audio (somehow I'm reluctant to call it hifi) it is very common to use 1F (1000000µF) caps in parallel with the battery to smooth out the peaks in power consumption of the amps. Those are usually rated for nominal 12V car electric systems so they can probably take 15V or so even though the label says 12V. That kind of caps are available on many stores that sell car audio stuff. They tend to be quite large anyway though.

[Paul Perry (Frostwave)]

you mean like this?

Thats what I planned on doing, but thanks anyway.

In your diagram, there should be a rectifier diode (say 1N4001) over on the left, between the power supply and the cap & resistor junction.
That way, when you turn the gear off, the cap can't discharge through the rest of the electronics, only thru the resistor and led.