Using piezo sensors as input

Using Arduino Sensors
1

I am looking to use piezo sensors are force sensors, and am having a hard time understanding how to interface it with my Arduino.

According to the official Arduino docs, only a 1M resistor in parallel w/ the sensor is required (http://www.arduino.cc/en/Tutorial/Knock).

But piezo sensors output AC waveforms, and thats not being accounted for in the example. What happens when the negative parts of the waveform enters the ATMega's ADC? Are they ignored, or can it cause problems?

I want to protect my ADC from damage, so I thought of also placing a 1N4007 diode in parallel with the sensor (in addition to the resistor) to only allow positive voltages through - but then theres the 1.0V voltage drop problem. Most of the small signals will not come through.

Any ideas?

2

The negative parts of the waveform will dissipate harmlessly through the protection diodes for such a high impedance source.

You can tie one side of the piezo/resistor combination to a 2.5V potential divider (1k to +5V, 1k to gnd) and the other end to the analog input - thus seeing the whole waveform.

If you want less sensitivity reduce the value of the 1M resistor.