Manually triggering an input from an electronic drumkit

Discussion in 'General Electronics Chat' started by Transatlantic, Jun 13, 2015.

  1. Transatlantic

    Thread Starter Member

    Feb 6, 2014
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    I have an electronic drumkit (Roland TD-8), and am wanting to trigger one of the inputs using a simple push to make switch.

    The pads themselves work using piezo transducers. I believe by hitting the pads, the vibrations are picked up by the piezo which then generates a small voltage, which the drum kit module picks up on. Depending on how hard the pad is hit, the higher the voltage generated by the piezo, and the louder the sample played.

    So - how could I go about triggering the module using a switch instead of the piezo? I'd like it to work such that when you press the switch, a small voltage is generated, but only for only brief moment.

    Like this :

    Untitled-2.png

    My basis knowledge leads me to something like this :

    upload_2015-6-13_16-0-3.png

    The LED would be replaced by 2 leads feeding into the drum module. This isn't what I want though, as I need it to be a push to make switch (so that it can be triggered very quickly, with minimal effort, using something like a small tactile switch). So I'm guessing the circuit could be redesigned using transistors to do what I want?

    I'm hoping to learn all this stuff but for the time being, was wondering if someone could mock up a circuit that does what I'm after?

    The other thing I'm not sure about is the amount of Voltage that should be produced.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What is the minimum time between successive presses of the switch?
     
  3. Transatlantic

    Thread Starter Member

    Feb 6, 2014
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    Probably about 100ms or so
     
  4. cornishlad

    Member

    Jul 31, 2013
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    Drum trigger signals are processed in the module for dynamics. Voltage is interpreted to volume out and pulse width to decay time.
    I'm not sure, but the voltage you are using (4.5v) may be right for max volume. But your 9v battery has a constant drain of 20Ma in that arrangement.
    Somewhere I remember seeing that pulse widths between 2us and 2Ms may be looked at for the decay value.
    Piezo sensors also output a series of declining oscillations, presumably at their natural frequency, which are also processed in a module to create the switching pulse. How a signal without those oscillations would be processed is another thing that might be an issue.
    Post your conclusions if you make it work.
     
  5. DickCappels

    Moderator

    Aug 21, 2008
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    If you use the button to put 4.5 volts a across resistor to ground to get a rectangular pulse (it will also have some milliseconds of "bounces" in the pulse) then pass the pulse through a differentiator network. An approximation of a differentiator is shown below. With the proper ratio of resistor values the negative pulse can be kept low.

    [​IMG]

    Suggested starting values for a time constant of 48 milliseconds (probably too long, but that's only a hunch):

    Resistor to the button to make the rectangular pulse: 22k
    Capacitor: 22 uf; positive end toward the button.
    Resistor in differentiator: 2.2k
     
  6. Transatlantic

    Thread Starter Member

    Feb 6, 2014
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    Thankyou! .. I've ordered some components to try this out. As an aside, the next step was to have the output pulse when the button is released too, so the negative pulse may actually be beneficial.
     
  7. DickCappels

    Moderator

    Aug 21, 2008
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    For a 10 Hz repetition rate drop that capacitor down ton2.2 uf.
     
  8. Transatlantic

    Thread Starter Member

    Feb 6, 2014
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    I've ordered a selection of different values to experiment with :)
     
  9. DickCappels

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    Aug 21, 2008
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    Good thinking.
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A switch-operated trigger will always produce the same amplitude output from the drum-synth. Is that what you want?
     
  11. Transatlantic

    Thread Starter Member

    Feb 6, 2014
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    Instead of hitting a drum pad to trigger the drum module (pressure picked up by piezo), I want to use a simple push switch to trigger the module. But as the module is designed to work with the input coming from a piezo, I was looking to have the push switch circuit output what the module expects.

    I assume you mean I won't get dynamic range, which yes, I am unfortunately aware of. Long story-short, I'm a paraplegic looking to play the drums, and as I have no use of my legs, am trying to create a bite switch so that I can operate the bass drum with my mouth.



    btw - still waiting on the components to arrive
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    O.k. Perhaps you could still use a piezo as the input device? I think if it were in a slim, flexible, waterproof enclosure it could behave similarly to one whacked by a drumstick, i.e would give some control over amplitude (but its output might need a bit more amplification).
     
  13. Transatlantic

    Thread Starter Member

    Feb 6, 2014
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    Yeah - I did try this, but I found that holding it in my mouth without having it falsely trigger was very difficult. To reduce the false triggers, I had to reduce the trigger sensitivity, which in turn reduced the dynamic range to pretty much zero.
     
  14. Transatlantic

    Thread Starter Member

    Feb 6, 2014
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    Thanks guys, the circuit appears to be working! .. athough I've noticed that I don't even need it!?, attaching a switch directly seems to work also. I swear I tried this at one point, and the triggering was intermittent, but my latest tests show it to trigger reliably.
     
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