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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.

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BACtrack S80 Specifiche Pagina 91