Triggering a Capacitive Prox. Sensor

Discussion in 'General Electronics Chat' started by Sensacell, Jun 19, 2012.

  1. Sensacell

    Thread Starter Well-Known Member

    Jun 19, 2012
    I am trying to develop a way to electronically simulate an object moving relative to a capacitive proximity sensor, a way to change the capacitance of a small (1 square inch) electrode under logic (TTL) control. The sensor would detect the change in capacitance and trigger, without anything actually moving mechanically. The response time needs to be on the order of 10 ms. The obvious solution would be to have a switch that would ground or float the 'stimulus' electrode, but it needs to be a very low capacitance switch to make the 'delta C' as large as possible.

    Any ideas?
  2. chuckey

    Well-Known Member

    Jun 4, 2007
    if you take a small capacitor (10 PF?), give one terminal as the sensor output, then if you switch the other end between different DC voltages, there will be a small charge transferred through the capacitor, which might simulate the effect of a small capacitor which has its value changed.
  3. Sensacell

    Thread Starter Well-Known Member

    Jun 19, 2012
    Thanks for the response.

    The Capacitive Proximity sensor I am working with averages the sensing measurement over a period of time to improve noise immunity, so a 'step' of charge injection would not trigger the sensor.

    The sensor utilizes small AC voltages in the 100's of Khz range applied to a sensing electrode, it looks for tiny (femptofarad) changes in capacitance.

    If the 'stimulus' electrode is just a floating, insulated plate in front of the sensor, a small AC signal will couple from the sensing electrode via the mutual capacitance. If the 'stimulus' electrode is now grounded, the apparent capacitance between the two electrodes would increase, now that the stimulus electrode can no longer follow the voltage on the sense electrode.
    The actual capacitance between the plates did not change, but the voltage across the capacitor did, so the sensing circuitry would see a lower impedance- and trigger.

    So I think what I need is a super low capacitance / leakage switch to ground?

    Reed relay? (I hate relays- big and expensive)
    Diode switch!