Easy to build foot-controlled wah

[Skidood]

This project uses a 3 foot length of 1/4" soft silicone rubber tubing connected to a pressure sensor inside the effect box.
The end of the tubing is plugged and the last 8 inches is doubled back on itself and held together with 2 small zip ties..the area between the zip ties is where you press down with your foot onto the doubled tubing to make your wah go "wah"  I found that doubling the tubing back will essentially double the pressure increase inside the tubing when you step down on it, giving you enough wah...single tube doesn't cut it....
Make sure the tubing doesn't kink at the very end.
I really doubt the diameter of the tubing is important, but I havent verified this. I used tubing with an internal diameter of around 1/8" and an external diameter of 1/4 " ...anything with a smaller I.D. would have a very hard time getting pushed onto the fitting on the sensor...
The wah effect I used is a Dr. Quack autowah, and the variable DC voltage coming out of the pressure sensor circuit is connected to the 22K resistor at the base of Q2.  The pressure sensor signal is boosted by an op amp  which gives you the "control voltage".
The sensor is a Honeywell 24PCAFA6G..range is 0-1 psi, output is 0-36 millivolts...will work with a 9V supply)
The op amp is actually an instrumentation amplifier---Burr-Brown INA122P, which is a CMOS device, so observe static precautions when handling...this chip works on a single 9V suply, just ground the V- pin.
I used a SPDT switch to switch the control voltage to Q2 (thru the 22K resistor) between the normal control voltage supplied by the Dr Quack circuit (taken from the wiper of the range pot) (this would be "normal autowah mode") and the output from the sensor circuit. (foot controlled mode)
The DC output from the sensor is amplified by the op amp to a usable level before it goes out to the wah effect.... Gain at the op amp is adjustable with a 10K pot ..mine ended up set to about 7Kohm.
I added a 10K resistor to ground off the output from the amplifier, this may not be necessary.
It works very good, the only thing I can't improve upon is the fact that for some stupid reason, the sensor output sits at around 16 millivolts with no pressure applied, (it is supposed to be 0) so the wah effect is just starting to wah before you even put your foot on the tubing. It just gives you a slight tone change...Ideally, the control voltage should be zero with no pressure applied, but I didn't want to build a DC level shifter circuit..regardless, the effect still works well, and is pretty darn neat if I do say so myself.

The datasheets for the op amp and the sensor are available at www.newark.com, which is where I ordered the parts.
The tubing should be available at a lab supply outlet, or maybe use windshield washer tubing.  

2muchtime

[wui223]

nice idea if u could post the pics it will be better.

[Skidood]

Yeah, I know, but I just got it wired up tonight.....the datasheets will show you the pinouts, it's not complicated to do on perfboard.

[wui223]

But just didnt see a pressure sensor before, i think the hardware parts always a challenge for Wah

[Skidood]

The pressure sensor is about 1 inch long by 1/2" wide, and stands off the board about 1/2 inch

[C Bradley]

I'll have to post a picture of my DIY wah soon. It's rather.... unique.   Gotta get the bugs worked out of it.

[Paul Marossy]

Interesting idea Skidood. I would like to see what this newfangled contraption looks like.  

[Skidood]

Back from the dead.....it's been a year..this effect worked really well...

http://noblesteed.net/tube.htm
http://noblesteed.net/text.htm