Specifications

84
Component
Current Draw (mA)
Display – backlight ON
70
Sensor
50
Wireless ON
95
Other components
700
Total
1,610 mA = 1.61 A
Table 3.3-1: Worst case current draw for portable unit.
There will be a need for multiple voltages within the units. While the logic can
generally accept a range of voltages, the sensor will have more stringent power
requirements, as will the other components such as the display. As such, it is
prudent to have multiple voltages. To accomplish this, multiple power sources
may be used. For example, several different batteries can be used to generate
each voltage (and power source). However, the disadvantages are numerous.
Not only would this consume more space within our portable housing, but it
would also create multiple parts that would drive up cost and decrease reliability.
As such, it is better to use a single battery. In order to generate multiple voltages,
voltage regulators will be used with this single battery. Since the highest voltage
necessary in the circuit will likely be 5Vdc, the battery should be able to supply
around 1.5Vdc higher than the output of the regulator. This is due to the
requirements of the regulators. Therefore, a minimum battery voltage output
needs to be 6.5Vdc. The 7.4Vdc battery profiled in section 2.11 would meet this
requirement. The next step down in battery voltages is 6V, which would not be
meeting the 6.5Vdc requirement.
3.3.1 Portable Unit Power Supply
The voltage regulators to be used have been profiled in section 2.3. At a
minimum, two voltages are needed within the portable unit, 3.3Vdc and 5Vdc.
Since one 7.4Vdc battery is used in the portable unit as the primary power
source, the voltage regulators will be connected to this battery in order to
regulate the two primary voltages necessary for the other components in the
device.