Darlington Pair Touch Switch meets Edward Holmes' Toy Piano

Discussion in 'The Projects Forum' started by SimonRogers, Sep 21, 2014.

  1. SimonRogers

    Thread Starter New Member

    Sep 17, 2014
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    Hi All,

    I've made Edward Holmes' 555 toy piano and it works great.

    http://forum.allaboutcircuits.com/threads/toy-piano-project.84399/

    However now I want to see if I can remove the necessity for a stylus. I'm sure this should be quite simple but I've spent many hours on it and to no avail.

    I've put together this circuit here: http://www.simple-electronics.com/2010/11/simple-touch-switch-using-transistor.html

    Independently, that circuit also works perfectly.

    When I try to combine the two circuits, that's where the problems arise. Thus far all I've managed to do is make the entire toy piano turn on and off through the darlington pair, but obviously that isn't very useful –– I need to be able to make the individual resistors complete.

    Could anyone be so kind as to explain how I could make this connect properly?

    [There is a reason i'm not just buying small touch pads, I want the connection to be formed by a darlington pair so that I can eventually print the keyboard keys out onto paper using conductive ink, allowing a tap of the paper to trigger a note.]

    Thanks,

    Simon
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    So you have one "simple touch switch using transistor" per note? 25 of them?

    Still, it's not gonna work as is: the connection made by the stylus is an AC signal;the current either travels down from R1 to the '555 pin 6, or up from C1 to '555 pin 7, as R1 is one end of the "charge"section, while pin 7 is the discharge pin.

    A transistor such as the 2N2222 only conducts current in one direction.
     
  3. SimonRogers

    Thread Starter New Member

    Sep 17, 2014
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    0
    Thanks for your reply. I was hoping to have the 25 resistors connected in series to the switch, so that there would just be one switch. If each resistor has a break in it's connection to ground, surely all of them could be attached to the same switch (albeit with a lot of wires).

    Can it not be wired so that the switch simply disrupts the connection between the resistors in series and the AC signal reaching the 555? I guess not. I'm sorry –– I'm a complete novice. Is there not a way to do what I'm trying to do?
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Yes it can "be wired so that the switch simply disrupts the connection between the resistors in series and the AC signal reaching the 555"
    Unfortunately, that is just what edwardholmes91 did with the stylus which is acting like a multi-position switch.

    To keep using such a touch switch it may be easier to use 1 '555 per switch, use the touch switch to turn on the '555 power, and tune each '555 for each note.

    As far as your parenthetical notes go (which I did not notice last time) a mechanical method of recording and playing back the notes will be a gigantic PITA to get it all working, and will be far more expensive then a digital method.
     
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