All the capacitors used are electrolytic.
B. Working:
The signal from the piezoelectric source is fullwave
rectified through a diode bridge D1. As the source
signal ramps up, charge transfers to electrolytic bucket
capacitor C1 whenever the source voltage overcomes the
voltage already supported by this capacitor (plus two diode
drops).
As C1 charges beyond 12.6 V (the Z1 breakdown
voltage plus the diode drop across the base-emitter junction
of Q1), Q1 is forced into conduction, in turn activating Q2
and latching Q1. With Q1 on, the high side of C1 now has a
current return path to ground and discharges through the
Maxim MAX666.
The regulator is biased to provide a stable +5 V
supply, as long as C1 has sufficient charge to produce a
valid regulator output voltage (Vout).
When Vout swings below approximately 4.5 V (as
set by R5 and R6), the low battery in pin (LBin on U1) is
pulled below its threshold, driving the low-battery out pin
(LBout) to ground momentarily. This negative pulse through
C3 turns Q1 Off, thus deactivating Q2 and renewing the C1
charging cycle.