Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 82

Vibration-Based
Piezoelectric
Wind Energy
Harvester

RF Antenna

Energy
Storage and
Supply Circuit

Voltage
Regulator
Circuit

RF Transmitter
Circuit

FIGURE 2.42
Photograph of the piezoelectric wind energy harvester system.

a 33- F capacitor until the preset voltage of around 8.8 V and is subsequently

discharged to the RF transmitter load. A detailed schematic diagram of the
piezoelectric wind energy harvester system is shown in Figure 2.43 . It can be
seen from the schematic diagram that the Q1 and Q2 power semiconductor
devices residing in the energy storage and supply system act like a control
switch that would initiate an on or off signal to the storage capacitor to release
the stored energy. The design of the energy storage circuit is adapted from a
similar circuit designed by the Massachusetts Institute of Technology (MIT)
for digital RFID (RF identification) of piezoelectric [81]. The significant im-
provement in the revised circuit discussed in this research work is that fewer
components have been used in the revised circuit while it can still operate to
deliver similar performance. One distinct difference between the proposed
design and the MIT design is the turn off process of Q1. In the proposed de-
sign, turning Q1 off is determined by the voltage drop across R3 instead of
the MIT method of transmitting a negative pulse from MAX666 through C3
to turn off Q1.
Initially, both Q1 and Q2 are off, so the ground lines of the voltage regula-
tor (MAX666) and the RF AM transmitter (AM-RT4-433FR) are disconnected
from C1. As C1 charges beyond the preset on voltage threshold of around
8.8 V (the preset voltage level is determined by the zener diode Z1 as 8.2 V
and the base emitter junction of Q1 as 0.6 V), the control switch Q1 turns