Illuminating LED from linear Hall-effect device

[David]

The "proof of concept" is showiing some promise.  I had a chance to do some work on this over the weekend and was encouraged by the results!  Empirical tests indicated that magnet motion within an inch from the sensor triggered results.  As to whether it's linear or not, I haven't gotten that far.

I put a magnet on a screw that formed the "heel stop" on my homebrew pedal.  This screw is about an inch above the pedal "floor".  The south pole of the magnet faces downward.  This causes voltage output of the Hall device to fall as the magnet gets closer.  I then mounted the Hall device on the pedal where the magnet touches.  Motion of the pedal definitely causes a progressive change in LED brightness.  I then rolled a homebrew optocoupler with an LED and a 100K LDR, but resistance change did not seem very linear.  However, it DID change, which is the important thing.

[David]

The prototype pedal apparently has a way to go.  It's really "glitchy" and seems to make a few huge changes during its travel.  This isn't going to cut it.  Guess I'll have to find a way to smooth its movement out before I go any further.

Sigh...  I was counting on metal-to-metal friction to hold the pedal in place.   

[brett]

Hi David
RE: seems to make a few huge changes during its travel

Try a weaker magnet (fridge magnet?).

I used a spring under the footplate.  It's rather unlike the rocker motion of a wah, but was good once I got used to it.
cheers

[David]

Hi David
RE: seems to make a few huge changes during its travel

Try a weaker magnet (fridge magnet?).

I used a spring under the footplate.  It's rather unlike the rocker motion of a wah, but was good once I got used to it.
cheers

Brett:

Those are really interesting ideas!  There are some things that need to be fleshed out, though:

1)  About the magnet:  this is a possibility.  I'm going to have to use a magnet that has a south pole, though.  I'm just using a resistor to "convert voltage to current".  If I use the north pole, absolutely nothing happens.  If I use the south pole, the LED goes dim as the magnet approaches the sensor (which I'm sure you're familiar with).

2)  About the spring:  If you do that, how does your pedal stay in place?  Or were you not after volume pedal or "%^&*ed wah" functionality?

3)  About the "glitchiness":  I was thinking I might be able to address this by changing my pedal mount design so the mount points are squeezed by some kind of smooth washer.  I'm thinking metal-to-metal friction (which right now holds my pedal in place) is causing my pedal to "stick" or "bind".  Any thoughts on this?

[David]

This is getting interesting...   

I was doing a little tinkering with my pedal.  I think I may have figured out how to get rid of at least some of the glitchiness.  How?  I...  uhhh...  loosened a couple of nuts.   

BIIIGG DIFFERENCE!  MUCH smoother action!  Hooked up a home-rolled optocoupler to it.  The numbers seem promising.  My test setup did not permit me to watch resistance throughout the pedal's travel.  I probably hit ten test points.  It seems to be enough data to warrant trying to see how it does as part of an actual wah circuit.

[David]

I cobbled together a voltage divider from my home-rolled opto and a 1K resistor.  Since my divider's resistances are hopelessly skewed, it's not a good test, but it's a valid test.  Better yet, IT WORKS!

Now I need to implement the 3-terminal Rock'n'Control.

[David]

If you find that you want/need to drive multiple LEDs, etc, and the sensor doesn't have enough current you can do (among other things):

Not to sound ungrateful, but how is that connection that looks like an X made?  Does that represent the 3-wire junction that it looks like it might represent?  Where would I connect the output from the Hall-effect sensor?

[David]

I have been royally frustrating myself trying to link a 3-pole Rock'n'Control to my Hall-effect pedal.  I get either 2 dark LEDs, one very bright and one very dark, or two very dim LEDs.  As yet, I have not figured out how to distribute the Hall-effect output between the two transistors.  I was ready to trash the project.  However...

I got one of those flashes of inspiration I love so much.  I should have a few minutes to check it out later in the week.