Digitech XP-??? using an external Expression Pedal

[mwynwood]

Hi guys,
I've been working on modding my XP-100 for a little while with a bunch of help form the people in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=58144.0
So this question is kinda related, but not really...

I have a question about the 'LED/photocell' combination used with the foot pedal.
Is it possible to replace it with a jack for an external expression pedal?
I'm not sure what value would be needed, or how you would make it toggle between the expression pedal (when it's plugged in) and the stock LED/photocell.

Any thoughts, advice or general comments appreciated
Thanks!

[Govmnt_Lacky]

Why would you replace the on-board expression pedal, that is proven to work just fine.... with an external one? I don't get the reasoning 

All you are doing is adding more to your pedalboard 

[mwynwood]

haha yeah, fair comments! It does seem crazy. 
The request actually came from a mate of mine, who had completely broken his pedal mechanism and was interested in possibly re-housing it and just using an external Expression Pedal.

[Govmnt_Lacky]

haha yeah, fair comments! It does seem crazy. 
The request actually came from a mate of mine, who had completely broken his pedal mechanism and was interested in possibly re-housing it and just using an external Expression Pedal.

What part is broken on it? I might be able to help you out 

PM me with the details

[mwynwood]

I'm not exactly sure, I haven't seen it, but he said that the expression pedal is permanently set at "0", and that when he looked he thinks that it's beyond repair.
He then asked if it's possible to just install a jack for an Expression Pedal so he can use that.

[mwynwood]

My idea would be to remove the photocell, work out what value it is and just connect in the expression pedal to where it was.
Should be pretty straight forward (in theory)

Can anyone see a reason why that might not work?
Hints, tips, thoughts, helpful suggestions and comments welcome

[digi2t]

I think that if we could see some pictures of the damage, we might be better suited to make some recommendations.

I'm just concerned that if you replace the LED/LDR combo with a pot, that it may introduce noise issues.

[Govmnt_Lacky]

Just to add to the discussion...

It is NOT an LDR that is on the board. It is a "Photosensitive Transistor."

http://www.osram-os.com/Graphics/XPic5/00101788_0.pdf/LPT%2080A,%20Lead%20(Pb)%20Free%20Product%20-%20RoHS%20Compliant.pdf

[digi2t]

Just to add to the discussion...

It is NOT an LDR that is on the board. It is a "Photosensitive Transistor."

http://www.osram-os.com/Graphics/XPic5/00101788_0.pdf/LPT%2080A,%20Lead%20(Pb)%20Free%20Product%20-%20RoHS%20Compliant.pdf

Well.... there you go. I'm going to bed less stoopid tonight. Thank you Brother G.

[Govmnt_Lacky]

And to add....

A quote from the interwebz:

"The brightness of the light shining on the phototransistor's base (B) terminal determines how much current it will allow to pass into its collector (C) terminal, and out through its emitter (E) terminal.  Brighter light results in more current; less-bright light results in less current."

In the XPs, the phototransistor is set up to have +5VDC going into the Collector. The Emitter is tied to VBias via a small-ish resistance and also is tapped directly (before the resistor to VBias) to be sent on to the rest of the circuit.

So... it looks to me like the phototransistor is actually acting like a variable resistor EXCEPT that it is providing a resistance between Vdd and VBias.

The hard part is to find a pot with the proper resistance, sweep, and taper to mimic the phototransistor's response to the LED 

[slacker]

Yeah that's how it works, according to this schematic  http://experimentalistsanonymous.com/diy/Schematics/Vibrato%20and%20Pitch%20Shift/Digitech%20XP100.pdf the photo transistor acts as the top half of a voltage divider with R21 being the bottom. This will give you a range of Voltages somewhere between 2.5 volts and 5 Volts. Someone with a working one could probably measure the range of voltages, that might give an idea how easy it would be to substitute a pot.
A better idea instead of replacing the photo transistor is probably to use a pot, maybe with a bit of additional circuitry to drive a LED pointing at the photo transistor. That would give you some scope for tweaking the brightness range of the LED and the sweep to match the original.