Hi,
I have a PVDF sensor, which works on the principle that when stressed, it will give an output voltage correspondingly due to the strain that develops, similar to a piezoelectric sensor.
When I give it a flick the voltage should vary from -5 to +5 v in a sinusoidal way which ultimately damps out. On connecting it to the arduino, I can only get a reading from 0 to +5v, the negative part does not show up.
How can this sinusoidal voltage be read by the arduino ?
Flick the sensor hard enough and it is possible you'll wipe out the processor input pin. Arduino pins cannot tolerate negative voltage or voltage above supply level, usually 5 volts.
In order to properly measure your sensor, you need to know the total span of the voltage and then apply an opamp circuit that will shift the full negative to positive voltage range of the sensor to 0 to 5 volts maximum. It might also be doable with just resistors but it's a matter of the sensor characteristics.
Once the signal is converted from bipolar to unipolar, the Arduino can measure it and it's a simple matter of mapping the resulting 0 to 1023 analog count range to your -/+ input voltage or pressure or whatever it is that you're trying to measure.
Can you provide a link to your specific sensor?
You are lucky that your arduino is still working. Never give negative voltage to arduino for longer time it will destroy the board like avr_fed has said. You need an op-amp, which can level shift the signal, so that it is compatible with arduino. There are so many op-amp in market, I assume you might need something like single suppy or dual supply it's all upto you. Goggle Level shifting circuits to arduino, it might help you understand. We are waiting on your datasheet.
Try a 2*10Megohm voltage divider between 5volt and ground, with the tap to A0,
and the piezo connected between A0 and ground.
Use a 100n cap between piezo and A0 if you worry about DC on the piezo.
That will give an A/D value of about 512 when idle, with positive and negative deviations.
Code can extract positive and/or negative peaks, and/or do something when a threshold is crossed.
Leo..