@Paulus: Ok. Get yourself a piezo with the wires attached, like this one.
I tend to get my piezos from disassembled drum toys. It is cheaper that way, but if you only need one, you can find them online with the wires for only .50USD or less. This one is a dollar.
If you get one without wires attached. Solder a black wire (ground) to the metal part of the disc, then solder a red wire (output signal voltage) to the ceramic disc.
First you will want to bread board and measure the signal strength. You do not want the signal to exceed 5V as these inputs are directly attached to the MCU.
Hook the black wire to your ground, and the red wire to the circuit you want to measure the strength of. You will want to put a resistor in parallel between the red and black wire. This value should be low, around 1 Mega Ohm. If you are exceeding 5V, change the resistor value until it is in the proper range.
Finally, put a protection diode in serial between the red wire and your measurement. You should not get negative voltages from a piezo, but I wouldn't take any chances. You will get a voltage drop across the diode, so repeat the steps above. If it is in the neighborhood of 4.3 to 4.7 V, you are in good shape.
While you are doing these tests, see how sensitive the piezo is to lighter and harder taps. If you get constant voltage output, you will always be able to trigger an envelope from the Mod Matrix, but if the signal strength changes, you may be able to still trigger the envelope with a high mod amount value. However, you can also hook the voltage to velocity in the mod matrix if it is a sensitive piezo. Conveniently, you will only take up one CV input with this piezo.
Now that you have done the measurements, attach it to your Shruthi!
First, solder the red wire to your protection diode. Next, solder the ground wire to the GND and the protection diode to the CV in of your choice. Finally, solder the proper value resistor in parallel between the ground connection on the board and the CV input you soldered the protection diode to.
Now if that sounds messy to you, and I assure you it is, you may want to solder all of this together on a piece of pref board, then simply run two wires to the connections on the Shruthi digital board. That way there won't be resistors and diodes in the way the next time you want to add a mod.
JUST TO BE CLEAR:
You DO NOT need to solder this component to the 5V line. When you tap or other wise cause a piezo to vibrate it will generate its own voltage.