Fiddling with the I/O voltages of a transistor touch switch

Discussion in 'The Projects Forum' started by SimonRogers, Oct 8, 2014.

  1. SimonRogers

    Thread Starter New Member

    Sep 17, 2014
    4
    0
    Hi all,

    I've built this circuit with some 447 transistors I had lying around:

    [​IMG]

    The problem is it's off voltage is 1.6V and it's on voltage (when a finger connects the two wires labelled 'touch plate') is 2V.

    I'm feeding these values through a comparator to make them a clean 0V and 5V respectively, but there is some lag before the 2V settles comfortably back to 1.6V, and I think this is causing the comparator to get a little confused and giving a funny output.

    So I either want to widen the gap between the O/I voltages, or speed up the 2V signal's fall back to 1.6V. Is this possible? What components could I go about changing?


    ___________________________________________________________________________


    If you're interested in why I want to do this, I'm trying to make a musical poster, very very cheaply, using conductive inks.

    So far I've constructed Jonathan Heathcote's ATtiny85 Piano: http://jhnet.co.uk/projects/attiny_piano#construction

    Here's my own:

    And in order to get this to work on paper, to my budget, I'm using transistors to create capacitive switches for each of the keys. That's the circuit you can at the top of this post. Because Jonathan's design is using the analogue pins of the ATtiny85, I'm using the comparator to try and give it acceptable voltages from my touch switch.

    So I've done this:



    But as you can see there is some unwanted noise, which I thiiiiiink is caused by my comparator getting confused due to the lag of the touch switch.
     
  2. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    It's off voltage is 1.6 volts where? At the base of the transistors or the collector? Across the LED?
     
  3. SimonRogers

    Thread Starter New Member

    Sep 17, 2014
    4
    0
    Sorry – across the LED, or in my circuits case the resistor that is then attached to the comparator through the inverting terminal.

    EDIT: I'll draw out my full circuit in a minute to clear things up.
     
  4. #12

    Expert

    Nov 30, 2010
    16,284
    6,797
    Try a 1 meg resistor from the base to ground to shut the transistors off completely when there is no stimulating current coming through the touch switch.
     
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