how to simulate a resistor ladder pushbutton with a knob?

Discussion in 'General Electronics Chat' started by Steve C, Mar 23, 2010.

  1. Steve C

    Thread Starter Active Member

    Nov 29, 2008
    86
    0
    Hi guys,

    I've got a resistor ladder in place that can not be removed. I'd prefer not to have to make fancy modifications, either. The circuitry is simple. If the ladder sees, say, 1k across the ladder, a function is performed. Works like a champ with pushbuttons.

    I'd like to do the same with a rotary knob if possible for faster action. so that quickly rotating a knob will "basically pulse" a 1k passive resistance across the ladder. Rotation in the opposite direction would do the same, at a different resistance for a different function.

    I don't think rotary encoders are set up to do this. I really want to avoid doing something voltage driven. If there is a mechanical rotary knob that will enable this function, it will save me an enormous amount of energy and effort.

    Are you guys aware of any products like this?

    Thanks,

    Steve
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why don't you use something like an octal or hex thumbwheel switch?
    If you need more than 3 or 4 bits resolution, just stack them.

    Here's a hex thumbwheel at Digikey:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=CKN5038-ND

    If you really want a rotary, they have a couple of those in inventory, but they are a non-stocked item:
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=450-1184-ND

    Mouser has some PCB mount that are a lot less:
    http://www.mouser.com/Electromechan...-vyke?P=1yzvr7m&Keyword=rotary+switch&FS=True
    You'd need a screwdriver to actuate them.

    Here's another possibility:
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMsfGXvUAbE5P3qjwUTwvxOfiomJ3FVio9U=

    Check out this Mouser catalog page:
    http://www.mouser.com/catalog/catalogUSD/641/1685.pdf
    PT65303 or PT36103 might work out for you.
     
    Last edited: Mar 23, 2010
  3. Steve C

    Thread Starter Active Member

    Nov 29, 2008
    86
    0
    Two issues that I might not understand clearly:

    Is there a way to change the resistance measured depending on the direction spun?

    Most important though, wouldn't this solution mean that the switch would apply a resistance to the ladder at rest, and be open for a brief moment between a position change? The way this ladder works, the switches short through a resistance when depressed. So I would need the opposite behavior through a rotary. The action of rotating the switch would briefly apply the resistance across the ladder.
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    You would need some extra circuitry, but it is possible.

    An encoder usually gives quadrature pulses out, so both lines pulse for either direction.

    You would need some simple logic to decode the direction and pulse one transistor or the other and simulate the two direction buttons.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    OK then, why not a simple break-before-make rotary switch of an arbitrary number of positions, leaving both ends open?

    Whatever resistance you want, just connect it in series with the common. Connect all of the individual poles together.
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    I believe the idea is that turning the control knob will repeatedly 'press' the Up or Down 'button' input to simulate rotary volume or tuning etc.
     
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