Specifications

81
Figure 38 MRF24WB0MA Power State diagram (Reprinted with permission
from MicroChip®)
3.3 Temperature and Relative Humidity
Sensor
In order to measure temperature and relative humidity, we have chosen to
implement the SHT21 temperature / relative humidity sensor made by Sensirion.
This was chosen because it measures both the temperature and humidity, thus
simplifying our design by eliminating the necessity for two sensors. The SHT21
is one of the newest sensors on the market and was initially released in a
complete version on January 21, 2010. At this time, the manufacturer is offering
free samples of this product. At a size of 3x3x1.1mm, the SHT21 is currently the
world‟s smallest temperature and relative humidity sensor. This is advantageous
to us since the sensor will be surface mounted onto a printed circuit board and
space will be at a premium. Sensirion has samples of the SHT21 available to the
public. We requested ten of these samples from the company.
3.3.1 Sensirion SHT21
The Sensirion SHT21 Humidity and Temperature Sensor is the latest humidity
and temperature sensor that Sensirion has added to its line. It has a 3x3mm foot
print, comes pre calibrated, and uses I
2
C protocol to communicate to a
microcontroller. The SHT21 has a normal operating range that varies from 20˚C
to 100˚C (68˚F to 212˚F) at about 8% relative humidity and from -20˚C to 60˚C (-
4˚F to 140˚F) at 90% relative humidity as shown in Figure 39. The range that our
device is designed to operate in is well within these parameters.