Joe's LED Trick

[mikabodcrane]

I read in earlier posts about Joe's LED trick,  but the links to it seem to be dead.  Can some explain Joe's LED trick or provide a live link.  Thanks

[Mihkel]

http://www.diystompboxes.com/analogalchemy/pedals/ledtrick.html

[mikabodcrane]

Thanks, but I need a little explination.  When the effect is bypassed the base of the transistor is connected to the effect out.  How does this shut th transistor off?  Is there some additional wiring I am not seering?

[RDV]

There's some explanation of how that sort of thing works in here.

RDV

[mikabodcrane]

Gotta thanks

I checked out your web site.  Are you using this trick on your boxes with a DPDT switch?  I'm using the fultone 3PDT switch on mine.  I thought going to the 3PDT switch would eliminate the pop contribution from the LEDs, but it doesn't.  I've greatly minimized it, but not eliminated it.  Maybe this transistor switching will take care of it.  Any advice from your experience in this area I would greatly appreciate.

[RDV]

Gotta thanks

I checked out your web site.  Are you using this trick on your boxes with a DPDT switch?  I'm using the fultone 3PDT switch on mine.  I thought going to the 3PDT switch would eliminate the pop contribution from the LEDs, but it doesn't.  I've greatly minimized it, but not eliminated it.  Maybe this transistor switching will take care of it.  Any advice from your experience in this area I would greatly appreciate.

You're welcome.
I use GEOFEX's Millenium 1+ or 3PDT switches. Depending on mood or what I have on hand(parts-wise). The Millenium 1+ pops less than using a 3PDT IMHO.

RDV

[Samuel]

Also I've found Joe's LED trick, while simple, is not quite as reliable as the millenium and requires tweaking - i've had them work fine on boosters and then be 'always on' with some high gain circuits...

[Samuel]

(no offense to joe, of course, who is great, but just sayin')

[mikabodcrane]

I'll tell you what helped with the 3DPT switch.  I split the current limiting resistor into two resistors, each with half the total value.  I then put a capacitor from between the resistors to ground.  Try it.  I still can get some smaller pops.  Have you totally eliminated pops on your pedals?

[mikabodcrane]

I understand now about Joe's trick.  I need to have the effect ouput tied to ground with a DC resistance when the effect is bypassed.  The article you linked indicates a low enough resistance to ground is required.  What value do you typically use in your boxes for this?

Thanks

[mojotron]

Wouldn't Joe's approach constently draw current through the 330k resistor when the effect was plugged in, but switched off by the footswitch?

I'm thinking of battery life with a continuous current draw would not be that long.

[mojotron]

Wouldn't Joe's approach constently draw current through the 330k resistor when the effect was plugged in, but switched off by the footswitch?

I'm thinking of battery life with a continuous current draw would not be that long.

[mikabodcrane]

Yes, it would constantly draw current even with the effect bypassed.  I would have to do a calculation on the circuit, but with a 330k ohm resistor, I would think the current draw would be in the tenths of a milliohm.  That is very small. Also a generally run my pedals from AC adaptor.

What is the thinking with using Joe's Trick or the Millennium Bypass with the 3PDT switch.  Does they offer any additional pop elinination on the 3PDT then without it.

[mojotron]

Yes, it would constantly draw current even with the effect bypassed.  I would have to do a calculation on the circuit, but with a 330k ohm resistor, I would think the current draw would be in the tenths of a milliohm.  That is very small. Also a generally run my pedals from AC adaptor.

What is the thinking with using Joe's Trick or the Millennium Bypass with the 3PDT switch.  Does they offer any additional pop elinination on the 3PDT then without it.

If going to ground that would be 27uA - I think that is significant, but if it eliminated the pop - that might be a good trade-off.

[Joe Davisson]

If using a battery, seems like having a decent-sized (100uF) filter cap would prevent the LED popping. Does it happen with a power supply or just a battery?

I forgot to add (on the page) that a high-brightness LED is best, to make up for some of the brightness loss. A Darlington transistor could also be used (ala RAT) but I thought the 2N3904 would be more convenient to get.

Pedals are always on anyway, as long as the input jack is plugged in. The extra trickle is barely a difference. It's very, very small: less than 0.00027 watts.

[R.G.]

As I mentioned before, pop with true bypass circuits where the effects already have pulldowns is almost always a power wiring problem, and usually a ground wiring problem.

What causes the pop is the sudden change in ground voltage as the several-ma change in the LED current flows through the ground wires. The wire resistance make this a tiny voltage change, which is then amplified by the effect. The worst case is the usual practice of switching battery power by a wire to a stereo input jack.

Anything that reduces the size of the current change, the speed of the current change, or the path of the current change will reduce the pop.
You can
(a) rewire the ground to an *output* stereo jack for ground switching
(b) slow the change in current with resistors and caps, so the change will be rejected by the effect circuit's low frequency rollofff
(c) change to a non-jack style of power switching - the circuit for that is at GEO.

Notice that mechanical switches like the 3PDT will be *worse* than semiconductor switches, as metal contacting metal changes from off to on instantaneously, while semiconductor switches are usually slower.

[mikabodcrane]

Joe

Do you mean a filter cap between +battery and ground, or between the + and ground on the DC supply jack, or at the LED dropping resistor as some have suggested? Right now my unit can use either 9Volt battery or AC adaptor.  I am thinking about eliminated the battery option and just going with the AC adaptor.

Gotta on the superbrights I'm already using them

Thanks

[mikabodcrane]

RG

Bare with me here please because of kinda new at this, but let me ask some questions

a.  Is this just related to the trick of switching the battery on using a stereo input jack, such that I am covered if I go just to AC adaptor and no passage through the input jack.
b. Is this a capacitor at the dropping resistor and parrallel with the LED? What roll off frequency do I want to use?

Thanks

[Joe Davisson]

The 100uF cap goes between + and - (well, after where the power switches at the stereo input jack). So basically it goes on the PCB. It will filter the battery or re-filter the power supply. The idea is that when the LED switches abruptly, the capacitor keeps the rest of the circuit steady. I'm not sure if this method actually works, but I'm interested in knowing either way.