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Darlington Pair Touch Switch meets Edward Holmes' Toy Piano

Discussion in 'General Electronics Discussion' started by SimonRogers, Sep 21, 2014.

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  1. SimonRogers


    Sep 21, 2014
    Hi All,

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

    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:

    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.]


  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Can you post the circuit for the piano here. I'm not signing up there just to see it.
  3. SimonRogers


    Sep 21, 2014
    Ha! Yes it's quite frustrating, sorry, I forgot I'd had to register to view it.

    Here you go:


    Attached Files:

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It's not going to work in that application because current needs to flow both ways through the connection between the stylus and pin 7.

    You could possibly use the transistors to switch a logic level to the control input of a 4066. This contains 4 logic level controlled switches. The issue here is that they have a non-zero "on" resistance which is voltage dependent. However they can operate up to 15V, so they are suitable for this circuit in other respects.

    Since it's unlikely that the circuit will actually be accuratley tuned, they might be quite OK.

    However, the 4066 has very high impedance inputs. You may be able to get away with connecting these to the touch switch and using a high value (say 4M7) resistor to pull the input to the opposite voltage level.
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    In case you're still looking, here's a design that should do what you want.


    I've only shown four keys - the bottom three, and the top one. The "JPn" items represent touch areas for fingers.

    I've used a 555 as the oscillator, against my better judgement, because a DIY electronic project just isn't a DIY electronic project unless it has a 555 in it. But I recommend using one of the enhanced 555 variants such as the ICM7555 or TLC555 or TS555, for slightly better performance.

    The key circuits are identical apart from the position in the chain of trimpots (VR1, VR2, VR3... VRn) that they connect into. When the finger touch pad is bridged with a finger with a resistance of less than a few hundred kilohms, it overcomes the weak pullup effect caused by the 1M resistor and pulls the gate of the P-channel MOSFET most of the way down to 0V, turning the MOSFET ON and connecting the positive supply rail to the appropriate point in the chain of trimpots. The zener diode is needed to protect the MOSFET, which can be easily destroyed by static electricity.

    The rightmost key circuit, when activated, connects the positive supply to the left end of VRn. This enables and starts up the oscillator, and the resistance of VRn determines the frequency, in conjunction with RF and CF.

    If a different key circuit is activated, more series trimpots from the chain are connected between the positive rail and the oscillator, so the frequency is lower.

    The oscillator's output is called "F" and it can be used externally, but the duty cycle varies depending on the pitch. A second output is generated by U2 operating as a divide-by-2. This frequency is half of the F frequency (it's called "F/2") and it has a duty cycle of exactly 50%.

    The circuit needs a stable, regulated supply. I suggest +12V but it will work at +9V as well.

    Tuning is done starting with the top note. Choose CF to get roughly the desired top note frequency with VRn set to about 35 kΩ. CF must be a high-quality capacitor with a low initial tolerance and a low temperature coefficient. RF should be a 1% metal film resistor.

    Once CF is chosen, adjust VRn for the correct pitch at the top note. Then adjust the trimpot to the left to get the right pitch for the note to the left. Continue all the way to the left hand end of the keyboard.

    Depending on the pitch range and CF value chosen, you may find that you can't get a full semitone of pitch difference between key n and key n-1 even with the corresponding trimpot set to maximum resistance. In this case you will either need to use higher value trimpots for that note (and probably all notes below it), or use a higher value for CF and readjust VRn.

    I've listed several alternatives for the P-channel MOSFETs. These are all available from and are listed in order of increasing cost. Other small P-channel MOSFETs will also be suitable. MOSFETs are static-sensitive and must be handled appropriately.

    If there's a problem with the oscillator running at a low frequency all the time, put a very high value resistor (e.g. several 10 MΩ resistors in series) between pin 7 of the 555 and the 0V rail.

    This design is not intended to be accurate.

    Good luck!
  6. BobK


    Jan 5, 2010
    There was a video of a guy (Russian) who build a keyboard using coins as capacitive touch sensors using just a PIC6F84, an old, not very capable 18 pin PIC. It was polyphonic and he played an amazing demo on the video. Now that is a touch keyboard project! I just spent 15 minutes looking and I cannot find the video on YouTube.

    KrisBlueNZ likes this.
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