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

Discussion in 'General Electronics Discussion' started by SimonRogers, Oct 8, 2014.

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  1. SimonRogers


    Sep 21, 2014
    Hi all,

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


    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:

    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. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    Try a pull down resistor from the base to 0 Volts. Try a 1 Meg Ohm and see if that works. If it's still too slow replace the 100K input resistor with a 10K and now use a 100K pull down from base to 0 Volts. The pull down goes after the base resistor. You need to show us the comparator circuit first but I guess it might need some hysteresis (positive feedback)
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