Help using an LDR in a wah pedal. Built, doesn't work.

[william]

 I just built myself a wah pedal using a Vactrol in place of the pot.  The Wah works with a normal potentiometer, but the circuit I used for the LDR does not.  Using the pot plugged into the LDR does nothing.

I wanted to build a wah that could be used with an external controller.  I still have things racked up and use a wireless so I wanted to keep the guts of the wah near the rack.  I need to use a single TRS cable to control both a relay for the switching and the LDR.  Below is the design I put into the wah.



It doesn't work.  I'm pretty new at designing things so does anyone have any ideas of a method to achieve my goal?  Do I simply have the values wrong.  The method used in the Technology of the Wah page on Geofx is not suitable for my needs as it would require all three wires of the cable for just the pot, and nothing for the relay.

[R.G.]

I'd have to see the circuit of the LDR inside the wah to know.

[R.G.]

I should mention that I have built remote-pedal wahs before, and they do work. So there is some detail that's keeping yours from running.

[william]

The Wah side of the LDR is exactly as in the Geofx page.  I tested the LDR to verify it was working and applying voltage to the LED side changed the resistance on the LDR side.

[R.G.]

Divide and conquer.

Remove the Vactrol LED from the transistor collector temporarily and sub in an ordinary LED. See if you can get the expression pedal to vary the light from the ordinary LED. If not, there's a problem in the expression pedal/transistor setup.  Find it and fix on it until you can vary the LED current.

Then sub the vactrol LED back in, and temporarily disconnect the LDR of the vactrol from the circuit. Clip your ohmmeter to the LDR, and wobble the now-known-working expression pedal setup. Does your ohmmeter move? If not, reverse the terminals of the LED side and try that.

If it still doesn't work, test just the Vactrol with a voltage source through a resistor  to limit current into the LED and verify that you can turn the LEd on and off by connecting/disconnecting the voltage/resistor. When you can get the LDR to vary by connecting the LED, note the polarity of the LED in the vactrol, and hook it back to the known-working expression setup.

If it still doesn't work take diagnostic voltages and write back.

[william]

I got the wah working with an external Univibe controller but the sweep is very minimal.  Rather than using a vactec I ended up using a LDR from radio shack and a bright yellow LED facing each other.  I'm now using the POT feeding the 2n2222 to adjust the LED's on point.  Any suggestions on how to adjust the sweep of the LDR?

[Lurco]

you are doing this in absolute darkness, i.e.: with no stray light reaching the LDR?

[william]

Yea, lights off in a box.