# Woodpecker Exp Input



## icedcreameyes (Oct 31, 2019)

Hi there, I have gone and modded my Woodpecker to add an expressioni input for the rate using a stereo switching jack. Everything works fine but the expression pedal is only effective over a very tiny range. I know that this is caused by the difference in resistance between the expression pedal (10k) and the pot in the pedal (100k). My question is how do I resolve this issue. I assume it's a common enough problem that there is a fairly simple solution... right?... maybe....?


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## phi1 (Oct 31, 2019)

Not an expert on this circuit, but I think you could adjust c6, c7, and c8 to get nice ranges for each of the toggle positions.


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## Chuck D. Bones (Nov 3, 2019)

The right way to do this is to use a 100K expression pedal.  You can try making R10 smaller to get more range out of the exp. pedal and C6-C8 bigger to bring the freq range back down, but if R10 gets too low, the oscillator won't work.  How low is too low?  Hard to tell from 2N6027 the datasheet.

There might be a way to do it using your 10K exp pedal.  First, can you verify that your exp pedal is wired like this?

Tip = one end of the pot
Ring = pot wiper
Sleeve = other end of the pot

Your exp jack on the Woodpecker must be isolated from the pedal case.  That means a plastic Neutrik-style jack.


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## icedcreameyes (Nov 3, 2019)

Thanks for the tips! I'd actually came to this conclusion too. I'm going to try modifying those components and see how I go tonight. Thanks for the help!


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## KevTom23 (Nov 23, 2020)

icedcreameyes said:


> Thanks for the tips! I'd actually came to this conclusion too. I'm going to try modifying those components and see how I go tonight. Thanks for the help!


Resurrecting an old post, sorry. How did this mod go?  Was it as simple as finding a 100k exp pedal and wiring a plastic stereo socket to the rate pot? Or was there more to it?


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## yazooligan (Nov 24, 2020)

KevTom23 said:


> Resurrecting an old post, sorry. How did this mod go?  Was it as simple as finding a 100k exp pedal and wiring a plastic stereo socket to the rate pot? Or was there more to it?


Agreed. The Woodpecker was my first build and I’d love to mod it!


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## JackOnFire (Jan 27, 2021)

I noticed that the EQD page recommends a Moog EP-2, which uses a 50K pot, as the expression pedal. There's another layout for the Hummingbird online which has different pot values, including B50K for Rate. Would changing out the pot values be out of the question?


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## xefned (Apr 15, 2021)

Sorry to bump an oldie, but this is the one thread in existence on expression modding a woodpecker.


icedcreameyes said:


> Hi there, I have gone and modded my Woodpecker to add an expressioni input for the rate using a stereo switching jack. Everything works fine but the expression pedal is only effective over a very tiny range. I know that this is caused by the difference in resistance between the expression pedal (10k) and the pot in the pedal (100k). My question is how do I resolve this issue. I assume it's a common enough problem that there is a fairly simple solution... right?... maybe....?



IceCreamEyes, did you mod it the same way Chuck Bones suggested with jack *RING* to wiper? And jack tip and sleeve to each end of the pot? Or did you wire it differently?

I ask because every schematic I can find for expression pedals shows that the *WIPER* goes to the *TIP*, as unlikely as that seems.
And the wiper is tied (through a PCB trace) to pin #1 of the rate pot. So would it even be necessary to wire the ring and tip with separate wires?

I drew it out, fired up the soldering iron, ... hesitated ... and searched for "pedalpcb + woodpecker + expression" and found this thread.

Now I'm very hesitant to proceed because I don't currently own an expression pedal and there's a good chance I'll have to unbox and rewire this after I get one. Oh well, it's the nature of DIY.


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## xefned (Apr 15, 2021)

Chuck D. Bones said:


> The right way to do this is to use a 100K expression pedal.  You can try making R10 smaller to get more range out of the exp. pedal and C6-C8 bigger to bring the freq range back down, but if R10 gets too low, the oscillator won't work.  How low is too low?  Hard to tell from 2N6027 the datasheet.
> 
> There might be a way to do it using your 10K exp pedal.  First, can you verify that your exp pedal is wired like this?
> 
> ...



Hi Chuck, thanks for posting this recipe. You obviously know a heckuva lot more than me so I'm going to give it a go with your instructions. Thanks!

P.S. — my guess would have been to:

 tie Rate Pot Pin #1 to the jack tip,
 leave the wiper alone since it's already continuous with pin #1 on the PCB
 tie Rate Pot Pin #3 to the jack ring,
 leave the jack sleeve disconnected since it doesn't appear to carry audio. (no need for shield grounding.)


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## xefned (Sep 3, 2021)

Don't know if IceCreamEyes is ever going to return to this thread, but the advice I've read is to *get a 50k expression pedal* rather than a 10k expression pedal for more flexibility with a greater range of devices.

I went with the Moog EP-3. $53 from Sweetwater.

*Short version:* I got it working only through extreme tenacity and a willingness to rewire the jack 7 times.

Through different combinations of Tip-Ring-Sleeve measurements, the Moog pedal would go from a low of 122Ω (measuring ring to sleeve) to a high of 46kΩ. But it would also measure 88kΩ high resistance from tip to sleeve, which is how I ended up wiring it which gives a much wider range. At 46k it worked but I couldn't get the rapid Hummingbird top speed flutter with it. I would say this mod is not worth doing at 46kΩ. At 88kΩ? Yes. Much more frenetic and cool.

I now have it wired very specifically for this Moog EP-3, with the bottom switch on "other" rather than "standard." If anyone is in this same boat and needs help, just DM me and I'll draw it up for this rather specialized scenario.

(Edit: Works with either mono or stereo cable for the expression pedal.)


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