Specifications

108
signal, so a capacitor is placed between the rectifier and the regulator in order to
further smooth the signal. Once smooth, the signal is now ready to be dropped
to the necessary voltage by passing through a voltage regulator. This process is
described in Figure 54 shown below.
Figure 54 24V AC to 3.3V DC conversion
4.4 Remote Sensing Unit
The remote sensing unit is designed to be mounted on the exterior of a building,
where there will not be a power source available. The remote sensing will
therefore be battery powered. The components inside the remote sensing are
the temperature and relative humidity sensor, the Zigbee wireless chip, and the
secondary microcontroller. The temperature and relative humidity sensor has a
typical operating voltage of 3V; the Zigbee wireless chip has a typical operating
voltage of 3.3V; and the secondary microcontroller has a typical operating
voltage of 3.3V. Table 33 shows a typical operating range for the components in
the remote sensing unit.
Component
Min Operating
Voltage (V)
Typical
Operating
Voltage (V)
Max Operating
Voltage (V)
Secondary
Microcontroller
1.8
3.3
3.6
Zigbee Wireless
Chip
2.4
3.3
3.6
Temperature and
Relative Humidity
Sensor
2.1
3
3.6
Table 33 Operating Voltage Range for Components of Remote Sensing Unit
Since the battery will be powering these three voltages, the amount of current
being drawn by each component needs to be considered. This is important
because a battery must be chosen that will be capable of providing the required
amount of current for an acceptable period of time. For our application the
batteries should not have to be changed on a regular basis, so a battery must be