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voltage to resistance convertor?

Discussion in 'Electronic Basics' started by Andrew Howard, Apr 14, 2005.

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  1. If I have a source of variable voltage ( a sweep generator for example),
    is there any way to create a source of varying resistance in proportion to
    the voltage?

    About the only way I can think of off the top of my head is a light/LDR
    combination, but I doubt that would be very efficient or accurate.

    Anyone have any better ideas? Preferably simpler ones, even if it is
    slightly less accurate. Or maybe someone knows of a resistance sweep

    Andrew Howard

  2. If your resistance range is not too large, and the
    signals you will impress across it is small enough,
    a quad MOSFET array can be made to perform
    that function with an op-amp controlling it.

    What input and output range to you need, over
    what frequency range, with what distortion?

    Another approach is to use a multiplier IC, hooked
    up as a divider, with voltage input and current fed
    back from the output to the input. But again, the
    required specs are necessary to proceed.

  3. Thanks for the quick reply!

    The resistance, I will need between 0 and 2k5.

    Frequency, between about 0.2Hz and about 20Hz.

    Distortion isn't really that important, I'll be using it to control
    something that isn't all that precise anyway.

    The input voltage can be between 0V and 9V. I can probably regulate it
    to anything within that.

    In case you are wondering, it is for a Guitar FX pedal I am semi-designing.

    Thanks again,
    Andrew Howard
  4. Guest

    Maxim and Microchip have digital potentiometers that may be useful.

  5. Ah... but are they available in Australia?

    I'll look into it though
    Andrew Howard
  6. John Fields

    John Fields Guest

  7. ....
    [MOSFET array cut as unlikely to be suitable.]
    There will be accuracy and current capacity issues with
    the low end of that range. What a designer will need
    to know, (at least), is: What currents or voltages will be
    applied to the resistance?
    That will certainly help.
    Right now, your requirement appears to be:
    1. Accept an input voltage, 'Vx', ranging between 0 V and 9 V,
    on one port, consisting of pins VxIn and VxRef.
    2. Provide a "resistance", 'Ro', on another port, consisting of
    pins RoA and RoB.
    3. If current is passed thru Ro, the voltage across it should
    be Vx * 2500/9, within some unknown tolerance. There is
    no presently known limit on what that current might be.
    4. If a voltage is applied across Ro, the current passing thru
    it should be 9/(2500 * Vx), within some unknown tolerance.
    There is no presently known limit on what that voltage might be.
    5. The accuracy implied by items 3 and 4 applies only over the
    frequency range 0.2 Hz to 20 Hz. Outside that range, any
    voltage or current result is acceptable.
    6. The connection, if any, between the Vx port and Ro port, is
    unspecified. They might be required to be isolated from each
    other for an unbounded voltage difference. Or it may be fine
    for either RoA or RoB to be connected to VxRef.

    I state the above to clarify what is missing. For any of the
    unknown bounds and limits, smaller will be easier and if
    it can be unipolar rather than having to be bipolar, the
    design may be simplified. Not having to isolate the ports
    will greatly simplify the problem.
    What sort of circuit is going to use the synthesized
    resistance? Can it accept a voltage or current instead?
    There may be an easier way to solve the real problem
    than synthesizing a resistance.
    You're welcome.

  8. This is the circuit that needs the variable resistance. It is the basis for
    a simple phaser. Apparently it works by using the fact that the emiiter is
    180 degrees out of phase with the collector (or something). The varying
    resistance has been labelled R1.
    + (9V)
    .-. .-.
    47K| | | |2K2
    | | | |
    '-' '-' 1uF
    | | +[/
    1uF | '----[|------'
    \] | |/ [\ | |
    IN o------|]---o---| BC548 | | 1uF
    /]+ | |> .-. | \]
    | .----' | |<-----|]----o OUT
    o | | | | |2K5 /]+
    | .-. .-. | '-'R1
    === 22K| | | | | | o
    GND | | | |2K2'---' |
    '-' '-' ===
    |-----' GND

    (created by AACircuit v1.28.5 beta 02/06/05

    I have decided that I would prefer a sine wave, if that makes any
    difference to anything. Both the phaser circuit and the sine wave generator
    will be using the same power supply.

    I am fairly new to this, so I am unsure about what you mean by having an
    isolate resistance, or the difference between unipolar and bi polar. I am
    guessing that unipolar means that it doesn't go below 0V (changing DC?) and
    that bipolar means otherwise (AC?). If so, wouldn't it be possible to bias
    an AC waveform into DC, or am I getting confused. I have not yet figured out
    what I would be using to generate the sine wave, so if anyone has any
    suggestions, that would be great.
    Also, I am not sure how to figure out the current needed, and I have
    never dealt with this type of circuit before, so I can't really guess. It
    would probably be in the low mA range though.

    Thanks for the help
    Andrew Howard
  9. Ban

    Ban Guest

    If you want to take digital potentiometer for this task, it should be
    possible, but it will require a SPI or similar control signal, not a voltage
    input. If you modify the circuit, you might come to a solution with a
    multiplyer, but this is not a trivial case either.
    You can use the LDR/LED approach, but the lowest resistance will not be 0,
    but maybe 400 to 1k, so you get less range into the clean sound, where the
    circuit acts merely as a follower. It can be tried out easily, there are
    even combinations in a 8DIP readily available, I do not remember the part
    numbers, but google should help.
    Some manufacturers make motor pots, I saw some surplus ones go for only 6
    bucks, but only 10k, not 2k5. (k is not in capital letters, BTW).
  10. The circuit looks faulty as you have drawn it.
    Look at the signal path from the collector, through a 1uF, to the wiper
    of R1, through a 1uF, to the output.
    That is a very low impedance path, practically a shortcircuit for AC
    signals. The collector signal will be at the output in full force, no
    matter how R1 is set.

    Now you want to mix into that signal the opposite phase from the
    emitter, by using R1, and what effect do you want to achieve by that?

    I think the signal from the emitter will hardly be stronger that the
    collector signal, so the result will be an attenuation of the signal,
    which could be achieved a lot simpler, if that is what you want.

    Maybe you have made a mistake in the drawing, so the connection between
    the collector output cap to the wiper of R1 should be erased?

    Then we will at least get something meaningful, a circuit which can
    move between positive phase, through nothing, to negative phase, by
    moving the wiper of R1.

    If we replace R1 with a pair of FET transistors we can voltage control
    how much positive or negative phase goes to the output. Or by using
    analog gates, type 4066 series, the same effect can be achieved.

    But basically I think the whole project seems badly concieved and
    designed. What you really need is to start over from the beginning,
    make it clear what you want to achieve, and ask for help to design a
    circuit to do that.
  11. Bob Masta

    Bob Masta Guest

    There is a nifty way to do this with plain old 4016 analog
    switches. You use the input voltage to control the duty
    cycle of a high-frequency oscillator. The 4016 (or 4066, etc)
    plus some minimum resistance is used as the variable
    resistor. The effective resistance (amount of current that
    flows for any voltage) is then proportional to the duty
    cycle. The beauty of this is that once you build up the
    PWM control circuit, all stages can be made to track
    very closely, and adding another stage only involves
    adding one more 4016 section.

    Don Lancaster discussed this in his CMOS Cookbook.
    I have used it (many years ago) to create a phaser
    effect with multiple stages to give more notches.
    All you have to worry about is getting the PWM
    clock above the audio range.

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  12. [Brasfield wrote:]
    I concur with Roger on the misconnection between
    the top end of the pot and its wiper.
    Sorry, but I cannot understand what you mean by that.
    Are you saying you want the resistance versus control
    voltage function to be a sinusoid? Or that you intend
    to use sinusoids as the control voltage?
    That pretty much resolves the isolation issue. The
    control voltage can be assumed to share the same
    ground with your "phaser".
    You've roughly got the bipolar/unipolar distinction, except
    bipolar means can have either of two signs and unipolar
    means can have only one sign or be zero. A bipolar signal
    could have DC content or not, and it is meaningful to speak
    of the AC content of a unipolar signal. The AC/DC concept
    and the unipolar/bipolar concept are not the same.
    There are many ways to do that. Digitally synthesized
    sine generators are pretty cheap these days.
    Yes. But I am not inclined to help go down the synthesized
    resistance route. (I concur with Roger on that as well.)

    It would be easier to build a voltage controlled "phaser"
    than to synthesize a voltage controlled resistance to put
    into that circuit you drew.

    For example, by differencing the output of two multipliers,
    one fed with (Vmax - Vcontrol) and the other fed with
    (Vcontrol), (where Vmax is the fullscale multiplier input),
    you would get the effect you are after. You can get a
    good multiplier in an 8 pin package.
  13. (Sheepishly), the same function can be accomplished with
    a single 4-quadrant multiplier. One input is the audio to be
    subjected to "phaser" manipulation, the other input is biased
    to be +/- fullscale at the extremes of the control input range,
    and the audio output is the multiplier output. With suitable
    biasing, it could all be done with a single supply and an 8-pin
    multiplier such as the AD835. See,,773_862_AD835,00.html

    You're welcome.
  14. Thanks for the information.

    I have decided in the end that it is getting too complex. I'll still
    probably tinker around with it, and try out your suggestion of the
    multiplier. I'll probably try and find complete circuits of DIY guitar FX
    pedals, at least until I gain a bit more knowledge on some of this stuff.

    I have learnt some things though, and learning about electronics is one of
    the main reasons I try to make guitar effects pedals.

    Thanks again,
    Andrew Howard
  15. You can easily find many schematics of phasers on the web.

    Try googling for
    guitar phaser schematic
    guitar phaser circuit diagram

    A few useful links for you:
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