World's Cheapest I2C (I-Squared-C) Adapter

Step 3: Dissecting the Cable

Four wires are needed for our I2C interface: +5V power, ground, serial data and serial clock. Pinouts for the various types of video ports can be found on Wikipedia or Pinouts.ru. Remember if using a VGA cable to find one with the full complement of pins; some only include a subset.

Cutting back the insulation and shielding from the outside of the cable, you'll likely find two groups of wires within. Thicker wires, or bundles of wires wrapped in additional shielding, usually carry the video signal. We're not interested in these and they can be trimmed back. Thinner, unshielded wires usually carry the DDC (I2C) signals among others. A multimeter or continuity tester can help in finding the correct four wires for your cable. Using a bare connector can be advantageous here, as you just need to solder four wires to the pins of interest.

A note on the +5V power: available current is very limited; about 50 mA according to the DDC specification. Most I2C devices sip just a tiny bit of current, so it should be possible to run several at once...but if using more than one or two LEDs (or other comparatively high-current devices), external power should be provided.