Hall sensors as a replacement for heavy duty wah-expression pedal pots

[cytt0rak]

Hello, I'm looking for a cheap alternative for heavy duty pots and I thought maybe I could use hall sensors like how they are used in high end joysticks. Have anybody tried this?

[EBK]

I can't help but feel that the "cheap" requirement will sabotage your efforts, but as long as you aren't also requiring "simple" or "easy", it is an interesting thought.  I see potential issues of reliability, sensitivity, non-linear scaling/mapping, mechanical mounting, calibration, interference....
I thought about doing something similar with optoelectronics once, but abandoned my efforts without making much progress.

[amptramp]

The problem with a potentiometer is that it has a sliding contact that wears out the resistive track.  If you could tolerate the feel of a wirewound pot, that could be an alternative.  But one alternative that does exist comes from the world of radio: the dual tuning capacitor:



The only sliding contact is the brass contact between the frame and the shaft and it shows almost no wear.  I can't find the link but I know someone has done a Wein bridge filter and used it as a wah pedal in the past.  It would have to be a high-impedance circuit for a wah to be tuned by a standard dual 365 pF capacitor, but it has been done with audio oscillators.  In fact a Wein bridge audio oscillator was the first product of Hewlett Packard.

A Hall-effect sensor operates by taking an electrical current flowing through a semiconductor sheet and forcing some of the current sideways in response to a magnetic field.  The rate at which the magnetic field changes with each millimetre of pedal travel is something you would have to work out using a magnetic circuit similar to a moving-magnet phono cartridge.  The problem with a moving magnet is it will attempt to move by itself to a position with the least magnetic reluctance so you may have to add some friction to keep it in position.

[Mark Hammer]

In fact, Roland/Boss did use a Hall sensor in one or more of their wahs.

http://archive.ampage.org/threads/0/fxgen/002519/Re_Why_isnt_DPDT_Stock_in_crybaby-5.html

[cytt0rak]

I can't help but feel that the "cheap" requirement will sabotage your efforts, but as long as you aren't also requiring "simple" or "easy", it is an interesting thought.  I see potential issues of reliability, sensitivity, non-linear scaling/mapping, mechanical mounting, calibration, interference....
I thought about doing something similar with optoelectronics once, but abandoned my efforts without making much progress.

I'm having reliability problems with pots I can find locally either I buy them off mouser with bulk and pay huge import taxes+shipping or find out new alternative for heavy duty pots. I'm not actually going for cheap route but make stuff affordable with highest possible reliability.

The problem with a potentiometer is that it has a sliding contact that wears out the resistive track.  If you could tolerate the feel of a wirewound pot, that could be an alternative.  But one alternative that does exist comes from the world of radio: the dual tuning capacitor:



The only sliding contact is the brass contact between the frame and the shaft and it shows almost no wear.  I can't find the link but I know someone has done a Wein bridge filter and used it as a wah pedal in the past.  It would have to be a high-impedance circuit for a wah to be tuned by a standard dual 365 pF capacitor, but it has been done with audio oscillators.  In fact a Wein bridge audio oscillator was the first product of Hewlett Packard.

A Hall-effect sensor operates by taking an electrical current flowing through a semiconductor sheet and forcing some of the current sideways in response to a magnetic field.  The rate at which the magnetic field changes with each millimetre of pedal travel is something you would have to work out using a magnetic circuit similar to a moving-magnet phono cartridge.  The problem with a moving magnet is it will attempt to move by itself to a position with the least magnetic reluctance so you may have to add some friction to keep it in position.

That thing could save me from the problems but I might have trouble finding them in my country. I'll definitely check that out once I come across one of it.

In fact, Roland/Boss did use a Hall sensor in one or more of their wahs.

http://archive.ampage.org/threads/0/fxgen/002519/Re_Why_isnt_DPDT_Stock_in_crybaby-5.html

Is there something Roland havent done yet 
I didnt dug in to that yet but I think their enclosure design is special for this to mount hall sensors etc. I have to find a way to mount hall sensors to standard VOX/Dunlop type of enclosures.

[anotherjim]

Had a Hall effect swell pedal once. That needed some circuitry to match the variation of the sensor to a usable scale for an OTA used as VCA. Maybe that could be modified to use a Vactrol instead of Wha pot?
It was a Robert Penfold design from 1994(ish) but I'm having trouble locating a scan of the project. Google keeps giving me fishing magazines - huh!?

[Mark Hammer]

My gut sense is that Hall sensors would need to be installed closer to the pivot point of any foot treadle, and NOT at the toe end.  You'd want any magnet to never be more than a reasonable distance from the sensor, to avoid any sudden on/off transitions.  The smaller the range/arc of rotation (which it would be nearer the pivot point), the easier it would be to calibrate the thing.

[ElectricDruid]

Don't forget Hall effect isn't the only way to go. I had a Yamaha electric organ which used an optical swell pedal - a cunning arrangement of a small bulb and a LDR with triangular fin connected to the pedal. It worked very well, and produced no noise, despite being nearly as old as I am and spending its life getting wobbled about.

HTH,
Tom

[runmikeyrun]

Don't forget Hall effect isn't the only way to go. I had a Yamaha electric organ which used an optical swell pedal - a cunning arrangement of a small bulb and a LDR with triangular fin connected to the pedal. It worked very well, and produced no noise, despite being nearly as old as I am and spending its life getting wobbled about.

HTH,
Tom

That's basically how Morley pedals work.  LDR that has the light from the LED varied by a cutout in a black card attached to the treadle.  You can change the sweep to a degree by changing the shape of the cutout for different tapers and even the amount of wah if you close or open the cutout to let more or less light through. 

Here is a Morley volume pedal that works on the same principle.  At the bottom are two LEDs on the side, pointing towards the cutouts on the black card, which are filled with green on the second photo.  On the other side of the card are LDRs.  As you sweep the treadle the black card moves and varies the position of the cutout, allowing more or less light through.  This photo is actually from a modification, which you can see on the right cutout that is jagged compared to the square on on the left.


Ham fisting my way through one build to the next since 2003.

[JebemMajke]

Arduino + proximity effect and digital pot?

[Les Turnbull]

i have a morley classic wah , what dose the mod do Mikey .

[duck_arse]

runmikeyrun - your images don't show this end, unless I remove the "s" from the "https....." in your URL's. this seems to be cropping up a fair bit, is it common to tapatalk-cdn users?