26-04-2014, 11:31 AM
Design of a Low Power Wireless Sensor Node for Environmental Monitoring
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Abstract
In recent years, the interest in Wireless Sensor Network(WSN) has been
growing dramatically. To meet this trend, a low power WSN mote, using MSP430
micro-controller and ADF7020 radio chip, was designed to carry out building energy
management and environmental monitoring. In this paper a practical design procedure
of the mote, including schematic, layout design and software programming, is provided
and the mote board is shown. The mote utilized multi-layer architecture of power layer,
wireless layer and sensor layer to enable flexibility. Temperature and relative humidity
were measured and transmitted wirelessly. Furthermore, the mote had achieved low
power consumption because of the use of the sleep mode of the micro-controller and
turning off the radio during idle.
Introduction
A wireless sensor network (WSN) usually consists
of micro-controller, radio frequency transceiver,
and sensor board and power source [1]. A WSN
node’s functionality is determined by the micro-
controller, which operates the sensor to sample
data by various commands.
Technological advances in areas like comput-
ing platform miniaturization, low power radio
transceivers and network self-organization proto-
cols have enabled the development of small, non-
intrusive and configurable sensor platforms, which
last for significant mount of time with very limited
power supplies, and form an information sampling
network [2]. WSN acts as a key factor in the rela-
tionship between digital and analogue world, not
only because of the sensor technology, but also the
implementation of sensor networks. Size and en-
ergy consumption are the most important factors
for WSN platform [3]. It is challenging to provide
long term power supply since the WSN mote is
usually powered by battery. Thus the power sup-
ply is a critical issue of the WSN mote.
Schematic Design:
Two external crystals (32.768 KHz and 8 MHz)
were implemented. The 8 MHz clock is used for
high processing speed application such as ADCs,
while 32.768 KHz will be applied for peripher-
als to save power. I2 C, SPI and UART pin-
out were available in the MSP430 micro-controller
schematic design.
Measurements of Temperature
and Humidity
Fig.9 and 10 illustrate the measurements of tem-
perature and relative humidity value. In terms
of temperature, the test was carried out in dif-
ferent environments including fridge, living room,
kitchen and central heating which had a temper-
ature range from 0 degree to 50 degree. During
the temperature testing, the sensor unit showed
the value of less than 5% error rate. As regards
to the relative humidity, the test was performed in
time scale of one day from 9 O’clock morning to 3
O’clock afternoon. The comparison between real
value and the sampled relative humidity had less
than 5% bias, which was acceptable.
Summary
A low power consumption wireless sensor mote,
with MSP430 micro-controller and ADF7020 ra-
dio, is developed for environmental monitoring.
The mote was designed by PADS and program-
ming by CCS in C. The mote samples temper-
ature and humidity measurements with the on-
board sensor. The results of the measurements
are precise compared with the reference values.
Meanwhile, the mote could transmit and receive
the data by ISM ban ADF7020 radio chip. The
peak current consumption is as low as 18mA for
0dBm transmission power and 3.3v power supply.