Specifications

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circuit, the focus on reducing the idle power draw of the control unit would not be
served by adding in LEDs which may remain on indefinitely.
Figure 3.3.2-1: Designed schematic for the control unit’s power supply
3.3.3 Power Supply Design Summary
Since the device is made to be portable in a variety of environments, and not
limited to a single, controlled location, power is an important concern. As
calculated, the circuit past the battery must be able to support a maximum draw
of 1.61A. Indeed, this is a significant amount, and would normally call for a
battery with a large capacity. However, due to the addition of a charging circuit
designed for a 12Vdc input, recharging is convenient. A simple adapter for a
vehicle’s cigarette lighter outlet can be used, or a commonly available adapter for
recharging in the home or office from an AC outlet.
3.4 Display
The Voog Handheld Breathalyzer requires a means by which to display data to
the user, in order to communicate test results, connection status, etc. One of the
simplest ways to communicate with an LCD display is a serial bus, where data is
packetized by the microcontroller, and synchronously driven to a connected
device. Because the development hardware lacks the ability to directly drive RS-
232 data, either I2C or SPI would work well for a small display. Because it is a
more simple design, I2C was chosen for this. However, because I2C relies on
connected devices to share the same logic levels, it was important to choose a
display that could communicate on the same level as the microcontroller at 3.3v.
For this, the Newhaven C0216CiZ was chosen for external communications.