From: jeedom74000@gmail.com   
      
   So I used a 9V battery to input a step, see pic here: https://flic.kr/p/2oonEe5   
      
   - In purple the input voltage to the piezo   
   - In yellow the PDs output voltage   
   - Time/div setting is 5ms on the scope   
      
   So I would say the Time Constant of the system is in the order of 10-20ms,   
   agree?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)