First and foremost I’d like to warn you that I’m not a native English speaker. Sorry in advance for my mistakes. And also I’m kinda a newbie in terms of electronics; so go easy on me please, I will try to be as understandable as I can.
I have a project to carry out dealing with piezo technology. I have to flash an LED using a piezo film. Here is the assembly put in place and it’s working (fortunately):
Some information regarding the components:
Op amp : LM358N
Q1: NPN transistor BC337-025G
Piezo film: don’t ask me about it, I don’t have any information. I took it from a device. However it generates within the assembly 50mv (for low pressure) to 1V (for high pressure)
I have some questions in regards of the circuit that I don’t quite understand.
1) I used to have a resistor right before the diode as you can see on the following illustration, but I got rid of it because it appears to be irrelevant. Having it or not makes no difference. Why?
2) When I squeeze the piezo (a pulse is generated) then maintain the pressure for some time and then release the piezo generates a positive pulse. Why not a negative pulse? I’d like to get rid of this second pulse that occurs when I maintain the pressure and then release (for brief pressure it doesn’t occur). I’ve added a Schmitt trigger right after the op amp on this purpose but I have the feeling that the sensitivity has reduced somehow. Do you have any suggestion?
3) The resistor of 560kΩ plays the role of a pull down resistor for me. Is it the case? The assembly is working for a resistor of 250 kΩ (and even less but requires high pressure) to 600 kΩ (perhaps a bit more). Why this gap? This resistor has an effect on the sensitivity, I can more or less guess why but would you like to explain it for me?
4) At last the main question: How to improve the sensitivity of the circuit?
Thanks in advance, I'd be truly grateful if you can help me out.