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Need to design a sensing circuit

  1. Mar 12, 2007 #1
    Hi iam looking for help with a college project i am doing, it is for sensing forces but the forces are only small. I am using a material called QTC( which changes from an insulator to a conductor when a force is applied to it). I am not doing electronics at college, so i only have a limited knowledge of electronics and circuits. How would i design a circuit that would switch on an LED at a certain predetermined force, then if the force is too high it would then switch on alternate LED. if this sounds at all possible could somebody suggest a circuit to use, as i have said i am a novice when it comes to electronics. I have come across schmitt triggers but i do not know how they work. thank you in advance.
  2. jcsd
  3. Mar 12, 2007 #2

    you could have two series circuits coming from a source like a battery...with one of those pressure switches under some padding...going into the schmitt trigger turning on LED A

    have the other pressure switch under some MORE padding under the first one, hooked up the same way. Might use a transistor to get the right Vcc for the Schmitt triggers.

    Then just do some physics calculations, experimentation, and tweaking until you have the right padding to close the pressure switches with the correct force

    That's how I'd do it but I'm a bootleg college student
  4. Mar 12, 2007 #3
    You can either use the solution suggested by Number2Pencil or you can get away with ONE opamp that does the job for you: it's called a window detector, it is like 2 Schmitt-triggers in one ;)

    Basically, window detector is a device which will trigger on two occasions which are predefined by a resistor network that surrounds the opamp. So basically you'll have two trigger levels with some margin (window) which you define by resistor-setting.
  5. Mar 12, 2007 #4


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    I would probably take the output from your sensor to two op amp comparitors. This way you wouldn't be loading you sensor.


    The comparitors will each have a threshold that you can set by your choice of resistances. (you could use a variable resistor; R1 in the linked circuit)

    Last edited by a moderator: Apr 22, 2017
  6. Mar 13, 2007 #5
    You could use a force depandant resistor or a load cell depending on what you are doing.
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