Using an mbed and piezo disc to determine frequency
You could try something like this: 







simulate this circuit &ndash; Schematic created using CircuitLab

The two resistors give you the offset at about half the supply voltage. The capacitor gives a low pass filter at about 1/3 Hz. R4 helps prevent damage to the op-amp from shocking the piezo. R3 provides a DC path for U1 bias current. U1 should be a low bias current type that will work on the provided single low supply voltage, it cannot be a TL081.