OBD II Car Computer

Using the OBD-II (On Board Diagnostics) protocol, we can read live information from a car`s computer with our NerdKits microcontroller kit, as explained by our guest star, Elena T. , MIT class of 2011. This uses the port under your steering wheel to receive all kinds of information, and is used by auto mechanics for troubleshooting the `Check Engi

ne" light, and for periodic emissions tests with onboard sensors. In this microcontroller project, we implement the Variable Pulse Width (VPW) protocol, which is generally found on General Motors cars. Please note that if you attempt this project, your car may have a different signaling protocol, which would not be compatible with our code. The car we used was a 1997 Chevy Cavalier. This was introduced soon after the 1996 law requiring US cars to have OBD-II buses, but there are not many sensors accessible from the data bus. On newer cars, you`d find oxygen sensors (allowing you to calculate MPG), but for our demo, we were able to measure: WARNING: Please note that by attempting this project, there are several risks to yourself and your property. This project involves working in the presence of car battery voltage, which can supply dangerous amounts of current and cause fire or electrical damage. This project involves working with your car computer, and while manufacturers do their best to make them robust, it is possible to irreversibly damage your car`s computer or the vehicle itself. And of course, if you`re the driver, you should be focusing on the road, and not some circuit and LCD on your dashboard. Optocouplers (or opto-isolators) provide a way to transmit information without creating a direct electrical connection. In a single small chip package, they have an LED and phototransistor. Current passed through the LED...

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