Hi! Thanks in advance for any help or suggestion. Please, I'd like to use piezo mic but for what I know it should be protected in order to avoid high voltage input due to excitation of the piezo. I know some circuits for limiting voltage in, like in picture. How anybody did work with piezo connected to Audio IN of Bela Board or add~ 1 2 or Analog Input pin (adc~ 3-10) ?

You don't necessarily need that active circuit. In many cases you'll be able to directly connect the piezo to Bela's input simply using resistors. A reasonably large series resistor (e.g.: 4k7) can be used to protect the input from any over voltage coming from the piezo. If connecting to an audio input, the series resistor is all you need. if connecting to an analog input, you'll also need to bias the input to around 2V. This can be achieved with a resistor divider down from 3.3V, e.g.:

         3v3 
          |
          R1 1M
          |
          |    4k7
piezo+ ---+----R3----+ Bela analog in
          |
          R2 1M5
          |
piezo- ---GND

where R1 and R2 provide about 1.98V biasing, while R3 provides protection.

If going the active preamp way, keep in mind that on Bela you shouldn't use 5V for anything noise-sensitive, like the resistor divider in your schematic, you should use 3.3V there, correspondingly reducing the value of the top 100k resisto. You can still use 5V to power the opamp.

    2 months later


    I can confirm:
    Bela Mini with Audio Multichannel Capelet and 8 Piezos connected as [Audio In] with 1Mohm to 3.3V is fully well working.
    Using Pure Data triggering with [bonk~] it's fine.
    Thank you.