Specifications

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capable of providing current for an extended period of time. The secondary
microcontroller draws 200mA max, the Zigbee chip draws 5mA max, and the
temperature and relative humidity sensor draws 100mA max.
The battery we have chosen is a 3.6V lithium battery to power the remote
sensing unit. The nominal capacity of the battery is 2.1Ah which should be
enough to power our unit for a sufficient amount of time. The temperature and
relative humidity sensor, secondary microcontroller, and the Zigbee wireless chip
all have “sleep” modes which are designed to reduce power consumption when
they are not being used. The remote sensing unit will not be continually reading
temperature and relative humidity, but rather will take readings at a time interval
set by the programmers (for example, once a minute), send that information to
the main control unit, then return to sleep mode until it is time to make another
reading. Implementing this method allows the remote sensing unit to consume
much less power over time. In order to connect the battery to the PCB, a surface
mount battery holder must be implemented. The battery holder has contacts that
will connect with the battery as well as contacts that will touch the PCB. This will
allow for power transfer from the battery to the PCB. The 3.6V of the battery
must be converted to 3.3V to be used by the components and this will be done
by a voltage regulator such as the one used in the main control unit. Since the
output of the battery is DC, no rectification will be necessary, and the output of
the battery can be inputted directly into the regulator.
4.5 Relays
Relays are what will turn on and off the high voltage and high current circuits that
power the compressors, air handlers, and dehumidifiers of the HVAC system that
our project will be controlling. The point of the relays is to be able to make the
decisions using our microcontroller which has very small voltage and current
outputs to control a circuit with higher voltage and current characteristics.
For the HVAC control system we have chosen to use an SRS-03VDC-SL relay
by Songle. The model is the SRS relay, the 03VDC represents the coil voltage,
S represents a sealed structure type, and the L represents a coil sensitivity of
0.36W. This relay could potentially be connected directly to the output of the
main microcontroller because the coil voltage of the relay is 3V. The coil voltage
is the amount of voltage required by the relay by the controlling circuit. When the
coil voltage requirement is met, the EMF created by the coil of wire is sufficient to
make the switch change from the “off” contact to the “on” contact. Therefore,
because the output voltage from the I/O pins on the main microcontroller is
between 3 and 3.5V the relay and the microcontroller could be connected directly
to one another. Table 34 shown below describes the key characteristics of the
SRS - 03VDC-SL Relay.