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Digital resistor/opto coupler

Discussion in 'Hobby Electronics' started by Lyndon, Jun 15, 2004.

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  1. Lyndon

    Lyndon Guest

    Hi guys,

    I have a microcontroller board that has a variable analog voltage
    output via a DAC. I want to use this variable voltage to control a
    potentiometer on another board. Is there any device that allows me to
    run the analog voltage output through two pins and get a variable
    resistance through another two pins?

    + DAC output ----- ------ + variable 5k resitance
    | |
    | |
    O O
    | |
    | |
    - DAC output ----- ------ - variable 5k resistance

    Thanks guys.

  2. Ken Taylor

    Ken Taylor Guest

    Are you stuck with using the DAC output? There's a lot of digital pot's
    available which may be a better option.

  3. mike diack

    mike diack Guest

    Otherwise you're stuck with Vactrols and opto-fets (not nearly as
    linear as digital pots, but they ARE isolated).
  4. Johnny

    Johnny Guest

    You could use a PWM output driving a opto-coupler, assuming you have a
    supply voltage on the isolated side. You will need to use a
    high-speed opto such as H11L1. If you use a low-pass filter on the
    output side of the opto coupler, you can get a analogue voltage
    output. Variable resistance is more difficult.

  5. There are a number of other ways of controlling a variable resistance
    from a digital signal. Check out Analog Devices TrimDACs, Xicor also
    have a range of digital pots. If you want to control the volume of
    an audio preamplifier, you could use TI's PGA2311.
    You could also use the output of the DAC to control a FET.
  6. Paul Bealing

    Paul Bealing Guest


    I built something a few years ago for a specific HVAC application which
    provided a isolated 0-100 ohm output of 256 steps form a 0-10 volt input.
    It was reasonably simple. The input signal changed little and
    infrequently so we used miniature relays to drive a precision resistor
    network. A small micro with ADC read the input voltage and used a lookup
    table and some calculations to adjust the curve before driving the relays.

    There may still be a few suppliers selling these off the shelf.

    For a higher resistance output you could probably use opto-fets in place
    of relays.

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