Here the piezo elements produce AC, So we want to rectify that. So I made a bridge rectifier for the rectification process using the  four 1n4007 diodes.

We know that rectifiers are used to convert AC to DC, but not a pure DC. There would be considerable AC component in their output, called ‘ripple’, in addition to the desired d.c. component.

Most sophisticated electronic systems need pure DC supply to drive, or power them. To construct a good power supply which gives pure DC output, we need to remove or filter out the AC component from the output of rectifiers. As a filter I used 220 micro farad, 25 volt electrolytic capacitor.