Continuity PCB design help!!!

Discussion in 'The Projects Forum' started by emilj726, Jan 17, 2013.

  1. emilj726

    Thread Starter Active Member

    Oct 1, 2010
    52
    2
    I am working on a Continuity type of PCB design.
    The basic idea is that I want to increase duty cycle on the PWM 250KHZ circuit when the 470pF cap touches a conductor.
    When Pin 3 of LM358 is higher than negative that means we are touching a plate, hence LED on.

    can anybody suggest something?
     
    Last edited: Jan 18, 2013
  2. emilj726

    Thread Starter Active Member

    Oct 1, 2010
    52
    2
    I guess no one has any suggestions.
    I just wanna know if there is better way that I can sense continuity to change the duty cycle on the square wave oscillator?
     
  3. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    It hasn't even been an hour since your first post!

    This is an international forum. People are sleeping while you are impatiently "waiting".

    A 555 can do this much easier....

    Though... so can a wire.... continuity is either there or not... unless you are trying for resistance, two terminals that complete a circuit when shorted is a continuity checker.
     
  4. emilj726

    Thread Starter Active Member

    Oct 1, 2010
    52
    2
    I'm sorry I should have named the thread differently.
    I have to use capacitive coupling because I need a analog out.
    Capacitance proportional to conductor diameter. The highter the capacitance the thicker the cable. the higher the capacitance the bigger the duty cycle.
     
  5. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    you are using PWM, a digital output. If you pass this through a low-pass filter, you'll get an analog output...
    If you are dealing with the circular cross-section...


    Your RC oscillator will vary with temperature, which will throw your measurements off...
    You voltage divider to create your voltage reference will also change quite a bit with both changing temperature and supply voltage...
     
  6. emilj726

    Thread Starter Active Member

    Oct 1, 2010
    52
    2
    I guess I am just wondering if anyone has any idea of how I can change the duty cycle of the circuit that I posted based on making contact with a conductor via capacitance coupling?

    Thanks!
     
Loading...