A disc that makes voltage
Bend a piezo disc and charge moves from one face to the other, so a voltage appears across its leads with no supply at all. The board drains that charge away in a tenth of a second or so, which is why a tap reads and a steady press does not: this block senses change, not weight.
A bend moves charge
Inside the black case is a brass plate with a thin ceramic layer bonded to one face. The ceramic is piezoelectric: squeeze or stretch it and the charges in it shift, so one face ends up with a little more of one kind and the other face with the other. Wires on the two faces then show a voltage between them. That is the whole sensor, and it is why the block needs no supply: the energy comes from your finger.
A bent plate is also a capacitor, two metal faces with the ceramic between them, so the charge stays put until something lets it go.
Press Play in both modes. Pressing bends the plate and SIGNAL jumps within a millisecond. Keep pressing and SIGNAL falls anyway, because the 1 MΩ on the board drains the charge whether the plate stays bent or not. Let go and the plate springs back, the charge moves the other way, and the board's diode holds that swing a few tenths of a volt below 0, which an ADC reads as 0.
What that means for a sketch
It senses change. A tap reads; a weight left on the disc reads for an instant and then reads 0. A knock, a footstep through a floor, a ball landing, a drum pad: all good. How heavy something is: not this part.
How high the hump goes depends on how hard and how fast the disc was bent, and on what it is fixed to. So a reading says roughly how hard, not exactly. That is still more than the TK28 knock sensor can say: its spring switch is either touching or not.
Which of pressing and letting go makes the hump depends on which way round the disc's leads are. A tap does both within a few milliseconds, and then the plate rings, bending back and forth many times, so every tap makes a hump either way. The figure's traces are the shape, not a measurement.
When it does not work
That is how it works. The press makes a charge once, the 1 MΩ drains it in a tenth of a second or so, and the reading returns to 0 V while your finger is still down. To sense a weight or a steady push, use a force sensor or a load cell, not a piezo.
Letting go moves the charge back the other way, which takes the disc's node below 0 V. The diode stops it a few tenths of a volt down and the ADC reads that as 0. Which of press and release makes the reading depends on which way round the disc's leads are; a tap does both, so a tap always reads.
No. SIGNAL rests at 0 V, held there by the 1 MΩ, so there is no middle level to find. What you choose is how big a rise counts as a tap, and that depends on how hard you tap and what the disc is fixed to.
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