# Bi-colour LED wiring?



## BurntFingers (Dec 1, 2020)

Bear with me a sec.

Grr.

So I've got a distortion pedal. I'd like to have 2 volume controls that you can switch between, kinda working like amp channels I guess. Set one quiet, set one loud and it's sort of an inbuilt boost (almost).

The easiest method I could think up was this:





Just your regular on/on SPDT footswitch there. Hopefully that's how to do it. 

But, could I use a red/green bi-colour LED that switches colour when the above switch is pushed, and still turns on/off with the main bypass footswitch? I don't know a great deal about switches or LED wiring so now's a good chance to brush up. Could anyone help me work out the wiring here?


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## benny_profane (Dec 1, 2020)

You'd want to use a DPDT footswitch. One pole would be used to change the volume control going to the output (as in your schematic above). The other pole of the switch would be used to select the LED color. If the bypass switching you're using turns on the LED by connecting the cathode to ground when the circuit is active, you'd want to get a common anode bi-color LED. the two cathodes are wired to the outside lugs of the switch, and the center lug goes to the pad/lug on the 3PDT that is normally used to connect to the LED cathode. The anode of the LED is wired with a CLR to a 9V power source.


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## BurntFingers (Dec 1, 2020)

benny_profane said:


> You'd want to use a DPDT footswitch. One pole would be used to change the volume control going to the output (as in your schematic above). The other pole of the switch would be used to select the LED color. If the bypass switching you're using turns on the LED by connecting the cathode to ground when the circuit is active, you'd want to get a common anode bi-color LED. the two cathodes are wired to the outside lugs of the switch, and the center lug goes to the pad/lug on the 3PDT that is normally used to connect to the LED cathode. The anode of the LED is wired with a CLR to a 9V power source.



Ya know, that's a lot more thinking than I was prepared for. Time to brew some coffee!

And find a dictionary.


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## BurntFingers (Dec 2, 2020)

benny_profane said:


> You'd want to use a DPDT footswitch. One pole would be used to change the volume control going to the output (as in your schematic above). The other pole of the switch would be used to select the LED color. If the bypass switching you're using turns on the LED by connecting the cathode to ground when the circuit is active, you'd want to get a common anode bi-color LED. the two cathodes are wired to the outside lugs of the switch, and the center lug goes to the pad/lug on the 3PDT that is normally used to connect to the LED cathode. The anode of the LED is wired with a CLR to a 9V power source.


Sorry to summon you back into this but this is what I've worked out according to your very helpful advice. Hopefully I translated that across ok to the schematic sections as follows.

Here's what I've got for the common anode led, and I'll wire B2 to the top left lug on the bypass 3pdt. The volume schematic hasn't changed, just there for reference. Look ok?


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## phi1 (Dec 2, 2020)

I’d rethink your volume pot switching scheme. The way you have it, if Vol 1 is set low and Vol 2 is set high, when you switch to vol2, there’s still a small resistance path from vol1 lug2 to ground, which will suck a lot of your volume.  The difference between vol1 and 2 would be slight, and they would be interactive.

better way would be have another pole if the switch that selects between lug2 of the pots. So, lug one for both pots would be grounded, but lugs 2&3 of the unused pot would be unconnected.

you can do all this with a 3PDT stomp (2 poles used for vol and 1 pole for LED).


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## BurntFingers (Dec 2, 2020)

phi1 said:


> I’d rethink your volume pot switching scheme. The way you have it, if Vol 1 is set low and Vol 2 is set high, when you switch to vol2, there’s still a small resistance path from vol1 lug2 to ground, which will suck a lot of your volume.  The difference between vol1 and 2 would be slight, and they would be interactive.
> 
> better way would be have another pole if the switch that selects between lug2 of the pots. So, lug one for both pots would be grounded, but lugs 2&3 of the unused pot would be unconnected.
> 
> you can do all this with a 3PDT stomp (2 poles used for vol and 1 pole for LED).


Okie dokie. Cheers for the advice.

So there'd be another pole after the pots looking like a Y on its side, one throw connected to the output of lug 2 of each pot, the pole going to circuit output? Basically mirroring the pole before it.

C1: vol 2, lug 2.
C2: PCB output
C3: vol 1, lug 2.


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## phi1 (Dec 2, 2020)

Yep that sounds right


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## BurntFingers (Dec 2, 2020)

phi1 said:


> Yep that sounds right


Sorry dude I was editing the post when you replied to make it a bit clearer


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## benny_profane (Dec 2, 2020)

That’s a good point, I hadn’t even considered that and just focused on the LED wiring scheme.


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## Robert (Dec 2, 2020)

Depending on the circuit you might not really need to switch lug 3 of the volume pot, you could keep lugs 1 and 3 of both pots connected and just switch between lug 2 of each pot.


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## BurntFingers (Dec 2, 2020)

Robert said:


> Depending on the circuit you might not really need to switch lug 3 of the volume pot, you could keep lugs 1 and 3 of both pots connected and just switch between lug 2 of each pot.



Ohh yeah, switching the outputs rather than the inputs. Clever thinking.

It's a distortion circuit using 2 x tl072, then a 3 band active tone stack. The volume pots immediately follow the coupling cap after the tone stack. 

Not sure if they changes anything but it's nothing too insane, has a lot of gain though. 

If the inputs of both pots are always active though will there be some voltage dividing action happening or interaction between them regardless?


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## phi1 (Dec 2, 2020)

There may be some small loss through the unused pot that way, but it shouldn’t be interactive. Athe higher the output impedance of the end of the circuit, the more loss i would think.

Should be ok, I’ve never wired anything like that, I just didn’t mention it because I figured you’d have no trouble using 2 poles of the 3PDT.


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## BurntFingers (Dec 2, 2020)

phi1 said:


> There may be some small loss through the unused pot that way, but it shouldn’t be interactive. Athe higher the output impedance of the end of the circuit, the more loss i would think.
> 
> Should be ok, I’ve never wired anything like that, I just didn’t mention it because I figured you’d have no trouble using 2 poles of the 3PDT.


The 3pdt solution does seem to be the safest as there's some speculation around switching the outputs and having conjoined inputs. 

I wonder what pedalpcb meant about it being circuit dependant.


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## BurntFingers (Dec 2, 2020)

This is what I've drawn out. All '1' lugs are for vol 2, all '3' lugs are vol 1.

Overthinking it maybe.


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## Robert (Dec 2, 2020)

BurntFingers said:


> I wonder what pedalpcb meant about it being circuit dependant.



Mainly quirky fuzzes or circuits with a passive tone control tacked onto the output might behave differently if you parallel the two volume pots.

I wouldn't hesitate on anything with a low impedance output stage (buffer, etc).   

I don't think there's a circuit here that switches the high side of the pot.... but there are several that select between two pots paralleled across the output.  Take a look at the Duocast schematic (for example), both pots are in parallel and the output is selected from one or the other.

Most (if not all?) 3PDT footswitches are break-before-make, so I'd be concerned about pops if you switch lug 3 of the pot because for a brief instant the output of the opamp is going to be unloaded and the output cap can build up a DC voltage.  (which could cause a pop at the _next_ instant when the other volume pot is brought back into the circuit)

Ever built a pedal that didn't have a pulldown resistor on the output?  They're quite prone to popping...   Each time the volume pots are swapped you're going to have a "no pulldown resistor" situation briefly.    

If you have a spare set of lugs on the 3PDT to sacrifice then you have nothing to lose.   Just keep this in mind if you _do _experience popping when switching.


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## BurntFingers (Dec 2, 2020)

Robert said:


> Mainly quirky fuzzes or circuits with a passive tone control tacked onto the output might behave differently if you parallel the two volume pots.
> 
> I wouldn't hesitate on anything with a low impedance output stage (buffer, etc).
> 
> ...


Well, I could easily add a 1m to ground at the output if that's the solution.

Your dpdt with the switch at lug 2 sounded great, but you didn't seem 100% it would be the solution in this application. A 3pdt has it's own problems, and it's own benefits. 

100% don't want popping though, especially not in a circuit with this much gain (officially a buttload) so if there's anything that can do what I'm looking to do quietly and without interruption then I'm all ears.


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