wah digitally controlled, resistor value

[Dimitree]

hi everyone,
I'm planning to build a small wah that could be remotely and digitally controlled.
I need this for some reasons:
1) I don't have a wah and I don't want to buy a wah shell/case because of 2)
2) Sometimes I have different expression pedals available, some of those are not the right value/taper for a wah, so just a remote wah won't be ok.
3) Maybe I could run a sequencer into it, sort of Seek Wah (if I'm not wrong), or also an envelope follower to make a sort of auto-wah, or also control it with midi, and so on..

I can easily build a small digital interface with a uC that "converts" my expression pedals into voltage, so I can even change its taper if I need, and with autocalibration I don't have to worry about the value of the expression pedals. But of course a digital uC can only outputs voltage (0V to 5V using PWM), so I need something to convert this variable voltage to a resistor useful for the wah circuit (usually 100k, right?).
What could I use? a digital pot? a photocell? is possible to find a 0 ohm -> 100k photocell? other things?

[Digital Larry]

Two ways leap to mind.

#1 Use a CPU with DAC current drive to the photocell's LED and use the resistor value to get LDR minimum resistance at maximum current.  LDR has a lag as the light turns off which can give the LFO a special character.  In other words, you can only go so fast.  Also minimum resistance is uhh.... a few hundred ohms (can't bother to look it up)

#3 Use a CPU with voltage output and bias circuitry to drive a FET derived "resistor".

What happened to #2?  Well... it's a sad story...

[g_u_e_s_t]

you could also use a digital pot, although im not a big fan of them.

another possibility is an lm13700, or some such voltage controlled amp.  the pot in a wah pedal just adjusts the feedback gain, so you could use a VCA to do the same thing.

a third option is 40kHz PWM to a transistor/resistor combo, so the resistor is only in the circuit for a certain percentage of the time.  the effective resistance becomes the resistor value divided by this time percentage.

[composition4]

I don't know if I'm missing something here, but why not set it up as a voltage divider - send +5V and GND to either end of the expression pedal pot, and connect the wiper to an analog input on a uC with analog in? Plenty of uCs with analog in

Jonathan

[Dimitree]

No the problem here is not to read a resistance, instead I need to output a variable resistance from the uC.
I think I'm going to use 2 photocells with 2 leds (only one would result in a 2 lugs resistor while I need 3 lugs). This seems like the cheapest and smallest way.

[potul]

It highly depends on the wah circuit you are planning to use.

If the circuit is using a pot simply as a variable resistor, it's easier because you only have to use a LED/LDR pair.
If your circuit uses the 3 legs of the pot .... it's very tricky. It's not trivial to get a voltage divider with 2 LED/LDR controlled by one voltage only. You must take into account that you need one resistor to go up while the other goes down. Maybe the simpler approach would be to get 2 voltages out of the uC and control the 2 LED/LDR independently.

Mat

[SISKO]

Check in GEO The techonlogy of wha pedals to find out how to connect a simple LDR to controll a wah pedal.

[David]

It highly depends on the wah circuit you are planning to use.

If the circuit is using a pot simply as a variable resistor, it's easier because you only have to use a LED/LDR pair.
If your circuit uses the 3 legs of the pot .... it's very tricky. It's not trivial to get a voltage divider with 2 LED/LDR controlled by one voltage only. You must take into account that you need one resistor to go up while the other goes down. Maybe the simpler approach would be to get 2 voltages out of the uC and control the 2 LED/LDR independently.

Mat

What's wrong with having one leg of the voltage divider be a small fixed resistor?

[ElectricDruid]

Using the uC PWM output to drive the LED/LDR directly is a good option. The LDR response is slow, and the PWM can be fast (easily fast enough) and you have essentially a variable LED. Since an LEDs response is more linear when fed by PWM than by a voltage or current, this is good option. Furthermore, some uCs have "half bridge" PWM outputs which are basically two PWM outputs that are anti-phase. E.g. one goes up while the other goes down - exactly what you're after. So set up your uC to read a 0-5V input voltage from whatever value expression pedal you like, and you're right - it won't matter much. I've used 10K to 100K with no problems. Then the uC converts the input voltage to a PWM output, and spits it out on two half bridge PWM channels driving two LEDs driving two LDRs. Bingo!

There are other ways of course, but for a wah I wouldn't recommend digipots (to likely to cause noise/stepping). A better option would be a OTA wired as a variable resistance, or alternatively a OTA-based wah filter circuit. I once built a nice LM13700-based 12dB autowah with LP and BP outputs. They have the advantage of being directly current controlled.

HTH,
Tom