Optical footswitching...

[earthtonesaudio]

One of these days I'm going to re-house my Fab Echo into a large enclosure and add a couple footswitches for tap-tempo and modulation, and when that day comes I'll need a couple momentary switches.  I figured it would be fun to try something new and so I want to use optical switching.  I already have the necessary LED/photodiodes waiting to be salvaged from some old computer mice and the electronics is no biggie, so it boils down to a hardware problem for me.

I'm imagining some variation on this: http://www.geofex.com/article_folders/alteractuator/alteractuator.htm
...but instead of activating a switch, the actuator blocks the light path between the LED and photodiode.  What I'm wondering is how to make sure ambient light can't get in, and what sort of mechanical issues might come up.

Thanks!

[R.G.]

The slick way is to get a opto-interruptor module. These have a matched IR LED and phototransistor in them mounted looking at one another across a small slot. Being IR, they're pretty immune to ambient light, and have good, fast and clean response. They're used extensively for position sensing, in the zillions in printers, copiers and other paper handling equipment. Mouser has them, but they're pretty cheap in the surplus market.

[earthtonesaudio]

I'll give those a try if my salvaged-mouse parts don't work.  There's still the mechanical actuator issue though, interfacing it with the optics... 

[earthtonesaudio]

Some questions for anyone who's used opto-interrupters:

Would a threaded rod like in the GEOFEX actuator be sufficient to block the beam or would the threads cause interference somehow?

I've noticed the interrupters are available with different slot widths.  So are they "tuned" to focus best at this width, or is it possible to put them farther apart (to accommodate a large, sturdy actuator, for instance)?

[frank_p]

One thing I used to do for robots using the captor (photodiode or photo-resistor) is placing it in a small box with a little hole (1 to 4mm diam) facing the captor.  This way no light source would come hitting the captor face, only the light coming from your dedicated light source.  You can even put this black-box-with-an-hole over the light source (light bulb, LED, etc.)  I used that on conveyors and work fine.  Play a bit with distances and hole diameters and with your switching transistor biasing and, voilà.

[earthtonesaudio]

Thanks Frank.  Good to hear there is some flexibility in the distance/focusing of these things.

[frank_p]

Also for the mechanical part I was thingking about that after I made the reply (if you wand to fidddle with the plunger idea):

- You want precisely "machined" outside diameter plunger shaft with just enough spacing in the retaining bushing of the plunger
- You want a bushing that will install easily and will not cost a fortune

I had a traumatolgy stretcher design projet that required a lot of hydraulic command pedals that I conceived and the solution our team decided to adopt were using pre-machined bushing and shafts.  If you use those, you will be relatively certain that the space required between the shaft diameter and the inside diameter of the bushing will be of standard value to avoid seizing and wear.   This way if your pushing with you foot is not straight, the force-moment (torque) will be minimised compared to a less precise gliding assembly.

Prefabricated bushing will be press-fit diameter in the hole you will drill in the Hammond box (they are made like that) and they have a flange to keep them from falling inside the hole.

This is an idea: in my application I used them in rotation and on both sidewalls of the retaining brackets so it was more sturdy.

If you go in the Mc Master Carr catalog you can find those:

Sleeve bushings:
http://www.mcmaster.com/#sleeve-bearings/=3ccyf7
You could perhaps also use drill bits guides as bushing:
http://www.mcmaster.com/#drill-bushings-and-liners/=3ccwu1
Precision shafts:
http://www.mcmaster.com/#rotary-shafts/=3ccz6o
Retainers ring:
http://www.mcmaster.com/#retaining-rings/=3cd24y

etc...
If you can have the Mc Master catalog just for generating ideas and doing brainstorming, it's a good "idea" tool.

Those are only some ideas and there are still work to think about for the whole assembly with a head and a spring.  The "main" bottleneck issue is to have correct tolerances to permit smooth gliding and prevent premature wear.  This is for plunger type action.  But I think that a hinge design "à la Boss" would perhaps be more rugged and less expensive to do...

H.F.

[earthtonesaudio]

Thanks Frank!  I keep coming back to the Boss style hinged cover as well... but while it would greatly simplify the actuator part, making the hinged bits would probably be an equal or greater challenge.   

I do have the McMaster catalog, but I hadn't thought to browse it for ideas yet.  Thanks!