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High level

I'm trying to use an electronic drum pedal as an electronic drum trigger. My specific e-drum unit has only one pedal input, so I need to repurpose a drum trigger input in order to use more than one pedal.

The main issue is the pedal uses a membrane potentiometer, while the drums use a plain old piezo sensor so the outputs don't match.

The Pedal (Membrane Pot)

The pedal has a resting voltage of 256mV and uses a 33.2K membrane potentiometer. As you press down, the resistance approaches 0 ohm and voltage goes to ~6mV. From my research I've determined this is likely a voltage divider circuit? I've mocked it up and messed with the resistor values to get an approximate idea of how it works numbers wise -- no idea if this is correct, so it's just to give you an idea of where my head is at:

Pedal Circuit Mockup

Here's a screengrab of an o-scope measurement of the curve from resting to fully pressed down: Pedal Voltage Curve

The Drum (Piezo)

The drum appears to have a resting voltage of 0V, and when the sensor is activated it produces a negative voltage between 0V and -600mV which depends on how hard you strike -- lower the voltage the harder you hit.

Here is a screengrab of the curve from resting thru the strike and back to resting:

Drum Voltage Curve

Summary

Basically I need to have the membrane potentiometer produce a negative voltage? Negative voltage is a new concept for me, so hopefully you can clarify what it actually means.

The naïve approach would be to slap a resistor in series to bring the pedal resting voltage down to 0V, and thus pressing it will produce negative voltage -- it makes sense to me from the standpoint of basic arithmetic but I'm sure things don't quite translate that way to the real world.

I'm a software engineer who is quite obviously green when it comes to electronics -- I think I know how to get the numbers, but not what they mean or how to manipulate them -- so any insight at all will be very helpful.

Thanks for your help!

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  • \$\begingroup\$ Use a DC-block capacitor in series with the output. A rather large value. \$\endgroup\$ – DKNguyen May 30 at 2:40
  • \$\begingroup\$ Tony Has a good answer posted for you. Be consitervy with your grounding and ground loops. I prefer single point grounding, sometimes it is incorrectly called star grounding. Star grounding is quite similar to a single-point ground connection, with the exception that the common grounding point appears in the middle of the PCB \$\endgroup\$ – Gil May 30 at 23:33
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The kick drum volume is velocity controlled for the same signal swing , while the piezo is amplitude controlled so the gain , offset and input velocity range needs to be defined then it will be easy to design. Slew rate is easily converted to Bandwidth then to filter response. But this is a high pass filter response or differentiator ( vs low pass ) to convert position rate of change on pot to velocity to match the piezo. Normally risetime is measured by 10% to 90% of full swing for this. So try to measure this for various uses to determine fastest and slowest range of Tr risetime, then 2nd step is to convert that to a signal conditioner.

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