This project shows how you can use the Netduino Plus to make a tweeting breathalyzer—a standalone breathalyzer that can post messages about the detected alcohol level to Twitter, using an inexpensive alcohol gas sensor.
The Netduino is an open source electronics platform based on a 32-bit microcontroller running the .NET Micro Framework. The Netduino Plus is similar to the original Netduino, but adds a built-in Ethernet controller and MicroSD slot. Since the Netduino Plus can connect directly to a network, it can independently communicate with Twitter’s API without being connected to a computer.
The MakerShield is a simple prototyping shield that is compatible with the standard Arduino and Netduino boards.
In this configuration, the MQ-3 alcohol gas sensor will output an analog voltage between 0 and 3.3V to indicate the amount of alcohol detected. This output will be connected to one of the Netduino’s analog input pins and read by its ADC.
While it would possible to convert the sensor’s output to a numeric BAC level, this would require careful calibration and would be prone to error. For this project, I will use approximate value ranges to determine which of several messages should be posted to Twitter. An approximate reading will be displayed on an RGB LED.
The RGB LED is the primary status indicator. During normal operation, it shows the level of alcohol, represented by colors ranging from green to red.
Three transistors are used to provide power to the RGB LED. The microcontroller used on the Netduino has a relatively low current limit per IO pin (around 8 mA for most pins) so it is generally not advised to drive LEDs (which can require 20-30 mA) directly from these pins. Using a transistor (or another LED driver) helps ensure that enough power will be made available to each LED without damaging the Netduino.
This page shows some common transistor circuits, including a few "transistor as a switch" circuits. Since the RGB LED I am using has a common cathode (low side) lead, I am using PNP transistors to switch the anode (high side) of each color.