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Piezo/Piezo to Piezo/Switch converter for Roland TD10 Drum module

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afedericojr

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Good day,

I had a need to run two separate piezo trigger cymbals into a TRS input that was expecting a piezo/switch signal. The Roland drum module was designed to work with a single drum pad where the head was the piezo and rim was the switch, so in order to use two separate cymbals that are piezo, it needs to be converted.
Keith Raper was kind and brilliant enough to get this to work for a Yamaha drum set, and many in the vdrum community have tried fitting this to a Roland, but there seems to be many failed attempts at perfection, and then the discussion dies off.

In this post, https://www.toontrack.com/forum/e-d...o-piezo-switch-adaptor-by-keith-raper/page-2/

PFozz commented with the following:

Actually, Keith Raper is the man of the situation :)
I'm formerly not into analog electronics but here's what I understand from the schematics :
.D1 and D2 won't allow a piezo xducer to make his buddy "buzz".
.Q1 acts as a triggered switch : basically, if there's enough current coming to its "base", Q1 allow what's coming from its collector to flow to its emitter.
.R1 limits the sensitivity (more the value is, less sensible will be the detection)
.R2 increases the sensitivity (more the value is, more sensible will be the detection)
In the 2nd circuit, the "main" piezo xducer determines the velocity of a hit, while the "aux" piezo xducer is use to trigger a kind of switch that determines which zone of your dual pad is played.
In circuits 1/3, if you're playing the first mono pad, its piezo transducer is used to determine the velocity and basta.
If you're playing the second pad, its piezo is used to determine the velocity of the hits and what's coming from this same piezo will make the circuit "link" the ring and sleeve parts of your socket.

That information is quite helpful, but I was wondering if someone could help me understand the circuit in full as this was the first circuit I have ever made, and have been learning quickly to the best of my ability and time availability.

The issue I am experiencing is that initially the cymbal triggering the RIM (which is converted from piezo to switch) was having mistriggers resulting in the sound being cut off prematurely.
Based on the attached schematic, it looked to me that the loop at D1 may not prevent flow back to the Variable resistor, so I inserted a diode between VR1 and D1 to keep the flow on that side. Now instead of triggers that are cut short, those same triggers no longer fire, so the only triggers I do get are clean sounds/signals, but it does not occur every time.

Actually a point of correction I should make is that not every trigger fire is clean, as sometimes tapping that cymbal lightly fires the other circuit path which is the head/piezo input.

I may be completely wrong, which is why I am here, but the loop looked to me that if the value is incorrect, it loops until dissipated to ground.
I also replaced the 10k resistor (R1) with a variable for more control. I will try putting that back to spec though and see if it cleans things up.

I have also tried replacing R2 with a 10M resistor, but it didn't seem to make a change. I will swap again and compare.

Earlier in that thread, it was stated that the switch is looking for a .7 volt signal. I think my next step which probably should have been my first, is to measure each cymbals output, and compare it to a stock OEM signal from a drum pad. Also, I can compare the stock RIM / switch signal to the output on the adapter I made. I only have a basic meter made by klein tools, so hopefully that is enough.

Any help is greatly appreciated.

Thank you
View attachment KeithRDTXpanderCheapB.pdf
 

afedericojr

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It is starting to make sense. I see I can measure the piezo output, so I compared it to the OEM drum, and also compared that to my circuit output for both head/rim (Tip/Ring respectively).
I noticed the values were off, where the switch / rim was too high.

The first thing I added was a diode between VR1 and D1’s points. This prevented the choking or cut-off behavior.

I then added a resister after D2 to eliminate false triggers where the rim triggered the head. I believe I added a 330k.

I replaced the 100k variable with a 330k as well. This caused the voltage to drop slightly more so that it triggers more reliably. I attempted to jump right to a 1M, but it reduced the sensitivity if I remember correctly. I may need to play with it a bit maybe in another path, to see where I need to increase resistance to lower the voltage so that it is consistently between .0 and .5 millivolts with a hard spike of 1 at max. Right now, I still seem to reach 1.9 or so.

It works well enough for casual playing, as long as I don’t attempt some sonic blast beat china riding ;-)

I will attempt to try to get the values reduced more appropriately at some point, but I have to learn more about how and why.
 

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