Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 83

MAX 666

R1
475 K
Q1
2N3906

D1 D2

V IN SENSE

V OUT2

SHDN

R4
150 K

C1
33 uF GND V SET

Z1
8.2 V
1N4738A

AC

C piezo

R3
940 K

R5
100 K Q2

IRF630

Piezoelectric
Wind Harvester

R2
740 K

D3 D4

Power Storage and Supply Circuit

HT12E

A0
A1
A2
A3
A4
A5
A6
A7
VSS

VDD
DOUT
OSC1
OSC2
/TE
AD11
AD10
AD9
AD8

AM-RT4-433FR

750 K

Vcc AN T

GND IN

RF Load

FIGURE 2.43
A schematic diagram of the piezoelectric wind energy harvester system.

on. The moment when Q1 is on, there is a voltage drop across R2 that is
higher than the threshold gate-source voltage V gs ( th )ofQ2inorder to acti-
vate the control switch Q2. Once Q2 is activated, Q1 is latched. This connects
the ground lines of MAX666 and AM-RT4-433FR with C1, allowing C1 to dis-
charge through the circuitry. MAX666 is a low-power series voltage regulator,
which produces a stable +3.3 V for the serial ID encoder (HT12E) and the RF
AM transmitter (AM-RT4-433FR) throughout the discharge of C1. When the
voltage across C1 drops below the off voltage threshold of around 4.58 V, the
voltage drop across R3 causes Q1 to turn off and hence in turn deactivates Q2
from the latched stage. When this happens, the ground lines of MAX666 and
AM-RT4-433FR are disconnected from C1, and the discharge of C1 is stopped.
Subsequent wind flow through the piezoelectric wind energy harvester in-
creases the voltage on C1, allowing the cycle to start afresh.

2.2.4 Experimental Results

In the experiments, the performances of the piezoelectric wind energy har-
vester system are explored and evaluated based on a wind speed of 6.7 m/s,
which is the preset threshold wind speed in Figure 2.40 to trigger the RF wire-
less transmitter. Whenever the wind speed reaches the preset wind speed of
6.7 m/s, the electrical energy stored in the capacitor is supplied to the RF
transmitter. The encoded digital information is then transmitted to the base