Vibration Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 131

Since the wireless control switch is designed for indoor conditions, the com-
munication range of the transmitter is set to be around 20 m, hence the power
consumption of the load read from Table 4.3 is 5 V and 1.41 mA, and the time
duration to transmit three 13-bit signals is 50 ms. Apart from obtaining the
power requirement of the RF transmitter load, the duration for a successful
transmission is also required to determine the total energy required. Thus, the
energy stored in the capacitor must be around 0.35 mJ to power the load at
more than 5 V for at least 55 ms. From these specifications, with reference to
Figures 4.20 and 4.21 , the 10- F capacitor is the most suitable value as it has a

peak voltage of 12 V and is able to store 0.83 mJ of energy when fully charged.
The principle of operation of the proposed energy harvesting system is de-
scribed as follows: Whenever the switch is depressed, the mechanical force is
applied onto the prestressed piezoelectric material, and some of the mechan-
ical energy is converted into electrical energy. The harvested electrical energy
from one single pressing is stored in an energy storage device until a certain
preset threshold level is reached before the stored energy is released through
a power management circuit to power up the RF transmitter. The RF receiver,
which is connected to the electrical appliance, receives the transmitted sig-
nal and switches the appliance on/off by controlling its power supply. The
schematic drawing of the proposed impact-based energy harvesting system
and its power management circuit is shown in Figure 4.22 .
Referring to Figure 4.22 , the operation of the circuit can be explained.
Charges generated by the piezoelectric transducer are first transferred into
capacitor C2, while the regulator and transmitter (load) are isolated by the N-
MOSFET (N-metal-oxide-semiconductor field-effect transistor) N-MOS Q2,
which cuts off the load ground from the source ground. The Zener diode Z1
disconnects the P-MOSFET (P-MOS) Q1 until the voltage across capacitor C1
exceeds its reverse breakdown voltage plus the threshold voltage. Once Q1 is
turned on, the voltage across R2, adjustable by the potential divider formed
by R1 and R2, exceeds the threshold voltage of Q2 and turns on N-MOS Q2.
Thus, the source ground and load ground are connected, and C2 starts dis-
charging to the regulator and the RF transmitter. R3 acts as the latch to ensure

V out

RT4
@433 MHz

V in

MAX666
5.6V O/P

R1

R'3

R'2

Q1

C2 R'1

Z1 R3

C1

I p

R osc

HT12E

Q2

R2

Piezoelectric Element

Storage/Latching Circuit

FIGURE 4.22
Schematic drawing of the proposed impact-based energy harvesting system.