# What is the role of the capacitor within the assembly

I have some questions regarding my assembly and especially on the role of the capacitor within the circuit.

Before I added the capacitor I encountered 2 issues:

• when the piezo was squeezed (a pulse is generated) then the pressure was maintained for some time and then released, the piezo generated a positive pulse that I wished to get rid of.
• when the pressure was maintained on the piezo film, I had such a sensitivity that the LED was still flashing. The brightness was weak though.

Adding the capacitor fixed these issues but I can’t explain why for the 1st one or rather I don’t know why. Of course, why I added the capacitor was for the 2nd point, I wanted to discharge this phenomenon through a capacitor. And it's working well.

And also I don’t get why since I’ve added the capacitor, the voltage seen on the oscillator is different. It’s pretty low now around 20mV while without the capacitor I have an input voltage of 20 mV to 800 mV depending on the pressure applied on the piezo film and an output voltage (right after the op amp) around 2.5V-2.8V.

• By the voltage on the "oscillator" do you mean the opamp? Did you look up a model for the Piezo? (A voltage source with a capacitor in series looked the simplest.) So the source capacitance is in series with the 0.1 uf.. and that's a voltage divider. – George Herold Jul 21 '14 at 12:38
• I mean I try to read on the oscillator the input voltage before the op amp and also before the npn transistor. With the capacitor in serie that doesn't solve my two issues that I spoke about. And sorry I don't have access to the model of the piezo. – Alibaba Jul 21 '14 at 14:01
• Hi Alibaba, In a previous post I suggested that you go on the web and find a model for your piezo. (So the electrical model of some thing might be a voltage source, or current source and then some other passive components, R's C's and/ or L's.) One model I found of a piezo said it looked like a voltage source with a capacitor in series. (Does that make any sense to you?) Once you have a model of your signal source it may be a lot easier to figure out how to make it work they way you'd like... or you'll see why it can't work and adjust you approach. – George Herold Jul 21 '14 at 14:40
• There is no oscillator in your diagram only an amplifier – Andy aka Jul 21 '14 at 16:22
• @Andyaka Was I supposed to show the oscillator on the diagram? It won't be part of the assembly at the end that's why I didn't see the point to put it there. I'm using it to check that my calculations match with the experiment. – Alibaba Jul 22 '14 at 8:21