2. Powering options

Obtaining power

• If your project requires a computer attached to the Arduino, then you don't need a power source for it. All that is needed is the 12V connection to the cable, which is readily available (on the OBD2 connector or wherever else next to a control module).
• If your project is independent from USB cables, the Arduino and cable will need 5V/3.3V power. The neatest solution is using a step-down converter module, but a hackier approach could be repurposing a car phone charger for 5V.
• Also, if your project is mostly computer-independent but you plan on connecting it to a computer occasionally (for debugging, for example), consider adding a switch in series to the power output of your regulator of choice, as powering an Arduino both from the USB cable and another source at the same time could have bad results.
• A worry for computer-independent projects connected permanently to 12V is of course parasitic power draw. If your project is installed permanently, consider powering from an S-Contact wire (switched positive = Terminal 15) instead of permanent 12V (Terminal 30).
  • Installing the project close to a control module has the added advantage of easy access to a switched positive terminal. The OBD2 port does not have such a connection.