The mechanical vibration sensor detects earthquakes and other very low frequencies up to 100Hz which is a low audio frequency. It is most sensitive at 40Hz which is a very low bass frequency. It does not detect higher audio frequencies.

A piezo sensor is sensitive at a few high frequencies (3kHz to 6kHz) and has poor sensitivity for frequencies less than 1kHz.
A piezo does not conduct then the image above is missing a biasing resistor for the +input of the opamp. Also it is missing a voltage clamp to prevent a high voltage generated by the piezo when it is struck from damaging the opamp.

 

@Audioguru again Ok so is it possible to manually tune the strings to be really floppy so that they vibrate at ~40 Hz? Or is that a bad solution?

If I'm going to use piezo sensors I then need to tension the strings to the point where they give off 3-6 kHz, which sounds like a lot considering this is not supposed to use tuners and such a robust construction..

 

I think someone suggested it.. a laser and target setup could work. The max deflection of the laser would correspond to amplitude. Seems like any setup that deflects from a baseline would do. I can think of many ways it could work despite the naysayers.

Jumping back to this solution; I really can't imagine how it would turn out. If I've understood @MrSalts 's reply:

how about using clear nylon strings, firing red or IR down each like a fiber optic. Then connect a time-of-flight sensor to the other end to read the time it takes to get to the other end. The string is stretched when plucked (and stretched more when plucked harder) so the time will be a bit longer for the light to get to the other end as the string is plucked harder and harder.

Having a ToF sensor only act as a sensor, not sending the lasers themselves? Would it be possible to fire the laser into the string, to the end of it (and possibly on a mirror edge?), reflected back through the string and then back to the ToF?

Edit:
Also, earlier on in the project I thought of using a "springed slide potentiometer" that would return to the middle as a default but could be dragged right/left to change its resistance. The string could then be fixed to the hat(?) of the pot so that when it's being struck, it drags the pot up/down back and forth as it vibrates. Ofcourse it will dampen the string but at least you can get a measurment of how hard the string had been struck.

I couldn't find any pot like it though, but what if you buy a slide pot and attach strings to it yourself? Then you could just measure the "position" like you do with any potentiometer.

 

If you are striking a drum that has the looks of a guitar then a piezo can be used like on an e-drum to trigger your sounds producer. A piezo is also used on an e-lighter to generate the high voltage spark when its hammer inside hits the piezo.

Use the amplified audio output of the guitar pickup to trigger your sounds producer.

 

Having a ToF sensor only act as a sensor, not sending the lasers themselves? Would it be possible to fire the laser into the string, to the end of it (and possibly on a mirror edge?), reflected back through the string and then back to the ToF?

there are some small modules and chips that do both. You'll need to split s monofilament line the long way (or melt two together) to make a Y. Then there should be enough natural reflection off of the end of the string (you get a reflection at any interface with two different refractive indices if the incidence angle is beyond the critical angle. The monofiment to air interface will be enough.
Zoom in on the photo of the chip, you can see the laser diode on one side and detector on the other.
https://www.st.com/en/imaging-and-photonics-solutions/time-of-flight-sensors.html
Note, that you'll need to select a chip with range about 250% of the length of your string because the range listed is for air, not monofilament plastic string.

EDIT. Rather than making a Y of monofilament, you can also run a string that is long enough to bring the un-tensioned extra length tail back to the sensor.

 

A different approach for sensing striking force would be an air-filled flexible tube attached to a pressure-sensor such as this.

 

A musician strums the strings of a guitar. A drummer might strike the geetar.