Optocoupling Strain Gauge

Discussion in 'The Projects Forum' started by shia_s, Jul 28, 2011.

  1. shia_s

    Thread Starter New Member

    Jul 7, 2011
    I am currently working in a project that using a lot of foil strain gauges, but I only have one signal conditioner and one wheatstone bridge.

    I heard that it is impossible to multiplex foil strain gauge, so I decided to use switches and turn it on-off alternately.

    This is the circuit I imagine:

    Since I need to turn it on-off rapidly (about 5KHz), I use optocoupler 4n25 instead of switch.

    This is the circuit for 4n25

    But somehow, it seems that 4n25 can't act as a switch properly (it cannot form short and open circuit at port 4 and 5). Did I make a mistake on using 4n25? and are there any other solution instead of using optocoupler as a switch?
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Did you consider the poor CTR of the 4N25? What's the LED current?
    What's the current through the strain gauge?

    Can you post the values of the resistors?

    Also, did you consider the rise time of the optocouplers output? Are you measuring the voltage on the bridge also with the uC?
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    4N25 type opto-couplers are digital devices, not analog like a strain gauge is. Strain gauges are analog devices, not digital like a 4N25 is.
    Depending upon your strain gauge resistance, you might be able to use JFET's as analog switches. They should be fine at 5kHz.
    Research "analog multiplexer" and "analog switch" for more ideas. Analog Devices is good at making these kinds of switches and has many to offer at reasonable prices.
    You will also need a sequencer to zip along at 5kHz cycling your gauges. You didn't state how many points you're measuring, but a microcontroller would be a very easy way to implement one.
    Do you have a method to capture/display your measurements every 200μs? This could be the biggest challenge.
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    most strain gauge applications require a warmup/stabalization period after the application of biasing current, as that current will cause some of the quickest (and non linear) change.