Help with bend sensor circuit

Discussion in 'The Projects Forum' started by madeintaiwan, Sep 13, 2010.

  1. madeintaiwan

    Thread Starter New Member

    Mar 16, 2010
    Hi all,

    What I'm attempting to do is use a bend sensor from Spectra Symbol to provide an output of 0-5v. I've pored over the various diagrams, looking mostly at this:


    which outlines how to use an op amp to provide a 0 - 5v swing in the output. I measure 11K to 22K fully bent.

    Here's my problem. I've wired and rewired that circuit on a breadboard and cannot get it to function properly. My questions for the forum are this;

    1. How do i provide a negative 5v Vref? Do I need another part in front of the op amp? It seems pretty important to be able to provide a negative Vref...
    2. The Op Amps I have are TS272s, LT1168s, LT1101 and a quad Op Amp, are any of these what I need? Do I need a rail to rail op amp?
    3. Do i need to provide the Op Amp with power? In each of the various diagrams I've only seen the inverting, non inverting and output connected, never Vcc or anything else.
    4. Really this is my first official Op Amp circuit so I I'd appreciate any additional advise.

    I'm open to getting any additional parts but I'd like to be able to solve this quandary. This will be for a gestural interface project using an Eobody 1:
  2. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    Have you provided it with the correct power supply levels?

    The circuit you have chosen to use requires dual (i.e. positive and negative) power supplies or else won't work.

    There are also other versions of the amplifying circuits that requires only a single supply in the data sheet of the bend sensor.

  3. John P

    AAC Fanatic!

    Oct 14, 2008
    I think you'd do better with a simple voltage follower made from a single supply op amp, like the venerable LM324 (the 1K resistor may not be the right value):
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  4. bertus


    Apr 5, 2008

    Do you have a datasheet of the bend resistor?

    You could use a constant current source and put the current through the resistor to measure its resitance.

  5. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    I was about to ask the same thing.

    We have some strain guages here that we instaled and the resistance variations forced us to use some high quality instrumentation amps. The strain guages were also in a bridge form so we needed 2 inputs for stability.
  6. John P

    AAC Fanatic!

    Oct 14, 2008
    But--a high quality instrumentation amplifier is exactly what you're meant to use with strain gauges!
  7. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    Thats what a bend sensor is so maybe he should be using an Instrumentation amp.