RPi Pressure Sensor using Piezo Transducer
I was able to measure a rate of change in pressure using a similar circuit to this. I replaced r2 in the circuit with a 40k pot, and after much calibration was able to detect the scale of pressure depending on the time values between high voltages (above1.3v) on the gpio pin. However, I achieved hardly any resolution due to horrendous noise problems, as well as the fact that the piezo is not a capacitor (as people commented) and I was essentially just scaling the piezo's voltage response. Basically, it works as a knock sensor, not as intended.