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DIY String Pots

Discussion in 'Electronic Design' started by gavspav, Mar 1, 2007.

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  1. gavspav

    gavspav Guest

    I am aiming to overcome my natural ineptitude and build an oversize
    midi controller.

    See this thread:
    http://groups.google.co.uk/group/re...nk=st&q=gavspav&rnum=1&hl=en#b5409ed3322eb6d2

    I am most likely going to use a 10 turn pot with a pulley attached to
    the shaft.
    I am thinking I will also need some kind of bearing to support the
    shaft of the pot and protect it from the sideways force of the belt/
    pulley arrangement.

    Can anyone point me towards finding pulleys and bearings that will fit
    on potentiometer shafts?

    I realize this is probably a riduculous question if you are in the
    know but I'm Googling round in circles (rs, cpc etc).

    Thanks,

    Gavin
    www.digitalfunfair.co.uk

    PS I was hoping to use String Pots but they seem to cost over £100
    each. I only need a low res device. Are there any other alternatives?
     
  2. http://www.sdp-si.com/
     
  3. John Barrett

    John Barrett Guest

    McMaster-Carr has a broad selection of pulleys and bearings, but based on the 10 turn pots that I know, you are going to have some work adapting something to the pot shaft... definitly feasable though... just a bit of a job :) Something along the lines of some angle and flat aluminum arranged line THIS:

    || ||
    [pully]==========================[pot shaft adapter]
    || ||
    ========= =========
    =======================================

    1.5" angle stock, with a piece of 1.5" flat as a baseplate... use 1/4" ID bearings with 3/4" OD flanged.. use 1/4-20 threaded rod for your shaft, and the pully of your choice with a 1/4" shaft -- use nuts with nylon locks to hold everything together (one on either side of the pully, and one for each of the 2 bearings supporting the shaft)

    for the pot shaft adapter -- I'm designing for one of those rectangular 10 turn pots with the small screw on one end -- use a 1/4" ID to 1/2" OD shaft collar -- using the tool of your choice -- cut a groove across one face and glue in a piece of piano wire that will fit in the screwdriver slot on your 10 turn pot.... attach the collar to the threaded rod leaving a little clear space behind the piano wire -- should drop right over the screw on the 10 turn pot... mount the pot any way you like to the aluminum flat such that its screw is aligned with the 1/4" threaded rod




    As an alternative -- you might try linear encoders -- I know the stuff from the industrial motion control market is expensive, but you can get the pieces needed to make one pretty cheap -- about $2 per inch -- at http://www.usdigital.com/products/lin/ -- plus the price of the optical encoder -- It looks like they can do lengths up to 34 inches off the shelf (I researched this a few months back and they CAN do much longer if needed) -- you can home brew a linear guide to suport the encoder and keep it aligned with the linear strip out of 1/4" alumimum rod stock and some delrin/acetal bushings from McMaster-Carr (more aluminum angle stock !!)

    Go with the 120 cpi strips with index mark.. The output from the optical encoder can be interfaced to a 16 bit up/down counter, and that used to drive a 16 bit D/A converter -- the index can be used to reset the counter when you reach the zero position -- make sure everything stays in sync -- go from there to make it have whatever effect you want on your circuit, or since you mentioned midi -- hook the encoder to a microcontroller and have it spit out midi directly !! I've got the code for the Atmel Atmega168 to read optical encoders if you are interested, and the midi protocol is well documented (or at least it was 10 years ago when I last worked on interfacing midi with microcontrollers :)
     
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