Im trying to make piezo working with arduino to read very little vibrations.
Piezo can detect these vibrations but changes in voltage are very small (0.1- 0.8 mV). So I tried to connect it with op amp and dc voltage bias to be able to detect it on arduino. That is my schematics:
I would like to amplify those .x milivolts by 1000 times to have reasonable reading on arduino. I have encountered many problems with that and spent hours trying different configurations with op amp and everytime I try I have different results. So here’s what I was doing and what i measured:
- I build schematics on breadboard with Rf = 10kohm and R3 = 1kOhm. Everything worked, my 0.x voltage changes became x mv changes.
2)I did the same but with 100kOhm as Rf resistor and again everything was fine.
3)I did the same but with 1MOhm as Rf resistor and I started to have problems. I had constant 2.3 V output and sensitivity was really same as with 100kOhm resistor.
So I put 500kOhm resistor and I got 0.9 V as constatnt output and it seemed to be working fine beside that “resting” output beeing high. But after measuring few times output was constantly and slowly growing to 5V. When I disconnected battery from op amp and plug it again output was again 0.9V and stable but after few measserement it was growing again. Why is that?
After few trials suddenly output was stable at 0.4V but sensitivity was terrible. Even with high pressure put on piezo voltage barely changes as if it wasn’t amplyfiyng.
I connected 100kOhm resistor again and it had very unstable output constantly changing between 0.9V and 1.5V. Also sensitivity was the same as with 10kOhm resistor and not 100kOhm.
So I connected 10kOhm resistor and had stable output at 0.4V but it wasn’t amplifying at all. I had same voltage directly from piezo as after op amp.
8 ) I changed op amp as I thought something broke but it was the same
I connected 100kOhm again and it was amplifying alright this time and had stable 1.3 output but after every touch of piezo it changed resting output to different value like 0.9V or 0.4V.
I connected 500kOhm Resistor and nothing changed
I tried many times more and I always have different results. Like there is constant 0.9 V output and I unplug und plug battery again voltage changes to 2.5V.
Everytime I checked if the circuit is all right, if there’s undesired resistors contact and etc.
What may be a reason of such random behaviour?
Why amplifying doesn’t change when I change resistor sometimes?
Also is amplifying by 1000x possible?