Turning voltage into resistance. Photoresistor my best bet? If so, help!

[benallison]

I have a DOD FX17 volume/wah pedal. What I want to do is use is as an expression pedal, for my Line 6 M9 (which like most digital units, needs a 10k linear pot for expression control).

Firstly... what are my options? Are there several approaches one could take? So far, it seems my best bet would be to use the 5v CV output from the FX17, to control an LED, to control a photoresistor. Would this be the easiest route? If so:

1) I need to translate 5v to 10k... but the resistance of a photoresistor is at maximum in darkness, and at 0 when it's receiving the appropriate amount of light. So, how would I be able to determine the necessary luminosity of an LED, and translate that to, I guess, "off" on the photo resistor?

2) I'd rather just get a premade optocoupler deal. Are they expensive? Where on earth would I get something like that.

[daverdave]

If you just want to use it as an expression pedal you could just use the pot in the pedal, or replace i with a 10k pot I'd have thought.

If you want to use an optoisolator then you'd have to look at the datat sheet for the optocoupler you want to use, they list the voltage ratings, turn on and off times. I'd say if you were using a home made ldr / led combo then it'd be more a matter of trial and error.

You could look up RG Keen's wah page, I can't remember the name, but that has a diagram of craig andertons remote wah, that could be close to what you want to do.

[benallison]

I'd totally sway the pot in the pedal if it used pots! It's a completely different technology. It's like, a pcb-mounted piece of mylar that works as a variable capacitor.

[anchovie]

The Line 6 expression pedal uses a 10K pot and a mono jack, so I'd suspect it's just attenuating a voltage that is sent out by the M9 and read back by the digital part. It might be possible to ignore the "send" line of the mono cable and just have the CV of the FX17 connected to the "receive" line, but this is pure speculation on my part as I know nothing about the M9. If the digital brain has a 3.3V supply then I expect it won't like having 5V on a control pin, so please don't try my "guess" without further information!

[benallison]

Other ideaers?

[Processaurus]

The Line 6 expression pedal uses a 10K pot and a mono jack, so I'd suspect it's just attenuating a voltage that is sent out by the M9 and read back by the digital part. It might be possible to ignore the "send" line of the mono cable and just have the CV of the FX17 connected to the "receive" line, but this is pure speculation on my part as I know nothing about the M9. If the digital brain has a 3.3V supply then I expect it won't like having 5V on a control pin, so please don't try my "guess" without further information!

You're on to it, I think.  The 10K variable resistor is just a middleman, to get the M13 to see a varying DC voltage at the expression pedal jack.  If the original poster hooked a 10K pot up to the M13 expression pedal jack, and read what was happening with a multi meter, it would be easy to adapt the DOD's CV to what the M13 wants to see.

It's been a while, but I tried that with a DL4, which I bet has the same system, and think there was about 3 volts coming out the jack.  Inside, it could be 3.3v feeding a voltage divider made of a fixed resistor in series with the 10K variable resistor that the pedal provides, to ground, and the voltage at the point between the pedal resistance and the fixed resistor is read by the processor.

[benallison]

Brilliant. Thanks all, this is WAS easier than doing the LDR thing.

So far, people seem to be saying, yes, ~3 - 3.3v is what the M9 wants as input.

So I just need to scale down the output of the FX17 to match.

KILLER!