Magnet Pot - replacement controll for use in Volume, Wah, Exp Pedals

[drolo]

Following this thread about alternative Wah controllers :

http://www.diystompboxes.com/smfforum/index.php?topic=103350.0

I though I could share a little circuit i have been working on that uses a magnet and  Linear Hall Effect sensor to control LED/LDR pairs in Volume, Wah, Exp treadle operated Pedals, in case someone would find it useful.



The Hall Sensor (U2) I use is an A1302EUA-T from Allegro. It can be easily found online and is relatively inexpensive. It is fed with steady 5V by a 7805 (U1) and puts out 2.5V when no magnetic field is applied.

Depending on the magnetic field polarity and strenght applied to it, the voltage will swing down to almost 0V or up to almost 5V.

I am using the magnetic field that makes it drop to 0. So I have a control voltage of about 0.1- 2.5V to feed my LED drivers.

The magnet (a strong neodym) is attached to the treadle with the hall sensor on the fix pedal part facing it. You need to find the right spot on the pedal in order for the magnetic field to just not affect the hall sensor anymore when your pedal is heel down. When you lower the pedal, the hall effect sensor senses more magnetic field and drops the voltage accordingly.

So this voltage feeds the 2 LED drivers that light or dim each LED group panning out of phase. Each LED is facing an LDR.

In my example I am using 3 LED/LDR pairs arranged as voltage divider pots for volume, wah and TRS expression out, plus one single LED/LDR pair for a mono expression out.

I have included 4 trimpots because in my simulation of the circuit, the range of the LED current went all over the place with slight variations of any of the values. Though in real life, tweaking the bias to have the best on and off values is quite easy.

I have this exact setup shown above mounted in an old Colorsouond wah i have restored. The wah switch selects between Volume and Wah mode. And 2 toggle switches activate the TRS and Mono Expression modes, deactivating the Wah/Volume circuit. I have included a simple FET buffer stage before the Volume circuit.

Before I rebuilt this Wah I had tried a different LED driver arrangement with 2 Op amps, but could never get it to work properly. (because i'm mostly a hacker ..)

I am still not 100% satisfied with the out of phase LED drivers. This week I have simulated this one, which seems a bit more stable and easier to bias:



Still need to breadboard it.

I guess the issue is mainly the number of LEDS I am driving and the current involved. I guess I should have a transistor per LED but that would be a lot of hassle ...

I also was looking into R.G. panning LEDS : http://www.geofex.com/FX_images/Panning%20LEDs.pdf

But could not get a circuit simulation to work in LTspice (but again i'm a hack ..) Anyway I find that 4 op amps is a bit overkill and think with R.G.'s arragement I would still need extra LED drivers to run as many LEDS.

If anyone has other ideas or suggestions, they are welome ;-)

P.S. at some point I thought that I had invented a wheel that did not exist yet but then found out that Artec had a wah working exactly like that ... oh well ..;-)

[deadastronaut]

nice one david:

ive never messed around with hall sensors....but i do have lots of neo magnets from my sustainer projects.

looks cool.

[drolo]

Thanks Rob

I looove magnets 

It works pretty well and is easier to implement mechanically than LED/LDR's with a shutter for example or comming up with a pot rotating device. I don't have a workshop or tools to machine metal at the moment so it was a perfect solution for me :-)

[artifus]

maybe some useful info here? http://omk.sourceforge.net/index.php?p=prototypes&s=mtr&l=en

[drolo]

maybe some useful info here? http://omk.sourceforge.net/index.php?p=prototypes&s=mtr&l=en

That's an interresting project there, way more complex than what i am doing.

[artifus]

it's way beyond me too but thought the hall effect info may be of interest.

[drolo]

Thanks, it is.
I think i got the Hall effect right now. The part I am always tweaking really is the LED drivers, drive me nuts 

[Gurner]

Hall effect sensors are cool little devices, I used one to make my own gaussmeter....


(is it obvious I needed it made quickly & never revisited to tart it up?!!)

...it gives me good, accurate readings of magnetic strength (Sustainers, Pickups etc) & magnetic polarity on my PC screen.

Same principle as drolo explained above ....only I then AtoD the Allegro hall sensor output, do a bit of maths on it & squirt out the results down a serial link to a PICkit2 programmer & then onwards to my PC's USB port.

Re that touchless hall effect keyboard...that ought to filed under "Let's climb a mountain 'cos it's there" category - I see no practical use for that whatsoever (other than a learning exercise!) - you've got to physically wear the magnets  

[artifus]

Re that touchless hall effect keyboard...that ought to filed under "climb a montain because it's there' category - I see no practical use for that whatsoever (other than a learning exercise!) - you've got to physically wear the magnets 

so gurner's out. any other dragons in the den interested? 

[drolo]

Hall effect sensors are cool little devices, I used one to make my own gaussmeter....


(is it obvious I needed it made quickly & never revisited to tart it up?!!)

...it gives me good, accurate readings of magnetic strength (Sustainers, Pickups etc) & magnetic polarity on my PC screen.

As a matter of fact, I was building a gaussmeter for my pickup winding excursions when I had the idea to use it in a wah :-)

[earthtonesaudio]

What you have: linear voltage within a known range
What you want: R1(a), R2(1-a) (where 'a' is the hall effect voltage)

Easiest solution: PWM + CMOS analog switch

[mistahead]

I've actually been considering neo magnet implants, obviously I am not a fan of them in my fingertips but hands nonetheless...

Maybe I need to think along a second set for therimin like control of... STUFF!

[digi2t]

I recently bought a Sonuus Wahoo, and I believe it uses something along these line for the pedal. There's no pot.

[drolo]

What you have: linear voltage within a known range
What you want: R1(a), R2(1-a) (where 'a' is the hall effect voltage)

Easiest solution: PWM + CMOS analog switch

I have never worked with CMOS switches. Could you use them acting as a pot, for example, in a classic Wah circuit without any particular issues?
Or did you mean to use the CMOS switch to drive the LED's?
Still I think it might be a bit overkill for this application and a bigger footprint then 3 transistors (and trimpots ..) but definitely an interresting option for more complex applications