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Voltage to resistance circuit

  1. Jan 18, 2012 #1
    Hi,

    i have this requirement where i have a voltage sensor and i want to convert it into equivalent resistance output, in the sense that i have a micro controller board which has been designed to read resistance sensors but now i have a voltage sensor and i want to interface to this board. could somebody suggest some schematic and also a direct IC it would be helpful. The schematic would help to understand and do it on my own and the direct IC would be helpful for the final design.

    thanks and regards,
    Satya
     
  2. jcsd
  3. Jan 18, 2012 #2
    by "voltage sensor" do you mean a sensor whose output is a voltage? What is the output impedance of the sensor?
     
  4. Jan 18, 2012 #3
    yes voltage sensor means a sensor which gives voltage as output. i really do not know the output impedance of the sensor. is it difficult to design the circuit without knowing the output impedance? i do not know could you please explain me.

    thanks and regards,
    satya
     
  5. Jan 18, 2012 #4

    sophiecentaur

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    You have asked for a 'schematic' but I think it is you who needs to provide a schematic of the circuit that you have and a proper definition of your problem. So far there is very little to go on. What you have written so far could be taken is so many different ways.
     
  6. Jan 18, 2012 #5
    Hi,

    Sorry for lacking clarity in the question. Now i have made a picture of the same thing i wanted to clarify.

    In the figure the top portion i have shown the resistance sensor with the microcontroller board this is the present configuration i am using, the microcontroller based hardware board is designed in such a way that it can able to read resistance only ( on the board it converts this resistance in to equivalent voltage and gives it to microcontroller by passing current). The microcontroller based hardware board is fixed and i am not suppose to change the circuit design.

    Now my new requirement is i have got a voltage based sensor as shown in the picture in the second row. Now i want to design an intermediate circuit board such that i can convert the voltage sensor output into a resistance and give at as input to the microcontroller based hardware board. could somebody guide me to design the circuit (to convert the voltage to resistance).

    i hope i made the question clear otherwise please let me know.

    thanks in advance,
    regards,
    satya
     

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  7. Jan 18, 2012 #6
    What does this resistance-outputting sensor sense?
     
  8. Jan 18, 2012 #7
    The resistance outputting sensor will sense the fuel level.

    thanks and regards,
    satya
     
  9. Jan 18, 2012 #8

    sophiecentaur

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    That's a bit clearer - thanks.

    It may be a good idea to look at the spec of the microcontroller board. That input must 'really' be a voltage or current into an amplifier and the manufacturers suggest (for a simple implementation) putting a resistor across the input in order to produce this wanted voltage (/ current). Look in detail at the input circuit. OR see if there are any Application Notes available for the board. I'm sure you can't be the only person to want to use an alternative form of input signal.
     
  10. Jan 18, 2012 #9
    It seems to me in order to measure resistance, the microcontroller must either be measuring the current through the resistance from a known voltage source or the voltage resulting from a known current flowing through the resistance - the former I think would be easier. For instance to measure the current through an unknown resistance from a known voltage source, all it would have to do is measure the voltage across a known resistor in series with the unknown resistor.

    Since the microcontroller is most likely measuring voltage, you need to design a circuit to output the same voltage it would see across its reference resistor.
     
  11. Jan 18, 2012 #10

    sophiecentaur

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    Let us know the Model Number of the board. We can look for ourselves.
     
  12. Jan 18, 2012 #11
    Can i use the circuit suggested in the link below
    http://www.maxim-ic.com/app-notes/index.mvp/id/3284

    I also found that the FET or BJT resistance varies according to the voltage applied. Do you suggest can i use this principle to achieve my requirement. i have never done this it will be a tough task for me but still i want to try.

    Thanks and regards,
    Satya
     
  13. Jan 19, 2012 #12
    Yes you are correct. FETs act like ohmic resistance when not in saturation.

    nMOSFET.CC.gif

    This is probably fastest, cheapest option, but will have to make your circuit very sensitive, you don't have much of a "window" of usable resistance.

    http://www.datasheetcatalog.org/datasheet/vishay/70598.pdf

    Link included, google for some more :)
     
  14. Jan 19, 2012 #13

    sophiecentaur

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    Being an Engineer and, consequently, a pragmatist, I would not go for your solution (except to prove that it could be done that way!).

    I should find out what the input circuit to the board consists of (can you just not look at the components and the tracks on the board?) and use my common sense to choose a voltage range and appropriate series resistance to drive that input with.

    The instructions for using that board will have been written for the convenience of users of one particular application. They make easy for that particular application and harder for another. Get inside the design and go for the more workmanlike solution to your problem.

    BTW, how would you provide the FET with the limited range of conditions needed for it to behave as a variable R, bearing in mind that it will be connected to an, as yet, unknown piece of circuit?
     
    Last edited: Jan 19, 2012
  15. Jan 19, 2012 #14

    jim hardy

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    so you have two sensors now, one that puts out a voltage
    and one that "puts out" a resistance?

    and it's measuring fuel level?

    i'm guessing this is a gas gage of some sort?

    like the float - rheostat in automobiles, or float - potentiometer?

    does it have two wires or three?

    if three wires it is probably a voltage diivider, and the microprocessor actually senses the fraction of applied voltage on its wiper. cars use around 1 to 5 volts for most sensors.

    if only two wires the microprocessor probably injects a small current and measures the resulting voltage.

    in either case ,
    you need to give the fellows more observations about the devices you have in hand.

    do you have a multimeter?
    what does the old sensor look like? how many wires? how many ohms between them when tank empty, half and 100% full?
    what does the new sensor look like? how many wires? how many volts does it make when fuel tank is empty, half and 100% full?

    let's hope the new voltage sensor isn't an embedded micro sending serial data.

    with no observations to work from, it's kinda like Mark Twain's observation - trying to make chicken soup from the shadow of an anemic chicken. :) :)

    by the way that Maxim device is really neat!!!
     
    Last edited: Jan 19, 2012
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