Hello!

I am currently constructing a cough-sensor in which I used a piezoeletric sensor (obtained from a beepin-card, I don't know any further info about it):
I now want to be able to use it to arduino.
But I have 2 problems:
1) The signal has low amplitude (200 mV). I have tried to amplify it with an OP07 and with a charge amplifier (as shown below) but I have failed!
The signal has an amplitude of around 200 mV, with a frequency ranging from 0Hz (lets assume) to 0.6 Hz.
What kind of amplifier (it has do be breadboard compatible!) can I use and what kind of montage?
2) How do I offset the signal? I have also tried a simple resistor summation circuit but it failed also!

Thanks in advance!

 

What values did you use in your circuit? What size is the piezo element? Is the piezo ceramic or film construction?
Finally, what is the lowest frequency that you must sense. The lower the frequency, the more difficult the problem.

 

What values did you use in your circuit? What size is the piezo element? Is the piezo ceramic or film construction?
Finally, what is the lowest frequency that you must sense. The lower the frequency, the more difficult the problem.

Heelo RichardO, thanks for your reply!

I don't quite remember the values I have used, but they were chosen approximately to a cut-off frequency that I don't remeber honestly!
About the piezoeletric, it is a piezo ceramic and it has a diameter of 1.3 cm (the crystal part).
About the lowest frequency, it have made a mistake! The frequency range is from 0.6Hz to like 5Hz!

Should the OP07 work just fine?
And what about the offset?

 

I am just guessing but I would not expect the piezo to have more than 10 nF of capacitance. See what that value does in you calculations.

 

I am just guessing but I would not expect the piezo to have more than 10 nF of capacitance. See what that value does in you calculations.

What do you mean? Those 10nF of capacitance have relevance in what?

 

What do you mean? Those 10nF of capacitance have relevance in what?

It is related to Q of the piezo in your schematic. To be honest, I don't know the exact relationship off the top of my head.

I think that the gain of your circuit will be roughly Cp/Cf where Cp is the capacitance of the piezo element.

 

Alright!
But it has nothing to do with the cut-off frequency right? And using this montage I will have frequencies from 0 to 4Hz? (asking cause u said that the lowest the lowest frequency, the more difficult!)

 

Alright!
But it has nothing to do with the cut-off frequency right? And using this montage I will have frequencies from 0 to 4Hz? (asking cause u said that the lowest the lowest frequency, the more difficult!)

Because capacitance is involved you will _never_ get gain at 0 Hz. Think of the gain of the circuit (ignoring R) as being the impedance of Cf divided by the impedance of Cp. At 0 Hz, the impedance of Cp is infinite, therefore, zero gain.

 

Because capacitance is involved you will _never_ get gain at 0 Hz. Think of the gain of the circuit (ignoring R) as being the impedance of Cf divided by the impedance of Cp. At 0 Hz, the impedance of Cp is infinite, therefore, zero gain.

Got it! Thank you very much!
Any idea how to offset the signal to 2.5V?
Also the OP07 says "The wide input voltage range of ±13 V minimum combined with a high CMRR of 106 dB (OP07E) and high input impedance provide high accuracy in the noninverting circuit configuration". Do I have troubles if I use noninverting configuration of the charge amplifier?

 

I have seen this montage on the internet, that supposedly is better than the charge amplifier.
Any thoughts?

 

I have seen this montage on the internet

Indeed. Audioguru over at EP gave you a link to that!

 

Yes I think I solved my issue!

Yeah! I think he solved the issue

I'll try it!