I'm contemplating making an ATmega-based MIDI pad controller. The pads in the Akai MPC/MPD are based on force-sensitive resistors, but I think piezos can work in a similar way. I'm posting this in the digital section, but my questions are mostly about the conditioning of the analog signal before it's fed into the ADC.
1. if I don't filter out the DC, will I be able to read a constant pressure on the pads and not just hits?
2. I'm looking at the schematics eDrum (http://www.edrum.info/files/analog8_v06.gif) analog part; since the pads are going to be identical, I feel pretty safe replacing the level pots with fixed resistors. What else can I drop from that circuit?
I think piezos will only give you a hit signal, not constant pressure... but I might be wrong.
Piezos are capacitors. In theory, constant pressure gives constant voltage. BUT....
_Any_ current load drains-off the voltage. So your buffer must have "infinite" input impedance, and the piezo must have "zero" leakage.
How "perfect" does it have to be? Assume 1,000pFd piezo capacitance. Assume 1Meg load (buffer and self-leakage). Self-voltage will decay 63% every 0.001 seconds. Assume 10,000pFd piezo capacitance and 10Meg load (buffer and self-leakage). Self-voltage will decay 63% every 0.1 seconds. Assume 1000Meg load... that's not realistic in muggy air around sweaty musicians without extreme damp-proofing.