Since the introduction of the I2C bus in the eighties by Philips it has been used many times to provide a simple connection between a PC and home-built circuits. This often made use of a simple interface connected to the parallel port of the PC. This now seems a bit dated, so we’ve designed a USB/I2C interface for you to build.Articles about the I2C bus have appeared several times in Elektor Electronics. We have even published designs for a whole range of modules that communicated with a PC via the I2C bus. At that time we used an ISA card to provide an I2C interface. Another type of interface that was frequently used was a simple circuit that connected to the parallel port.But with the introduction of modern operating systems it became much more difficult to control the parallel port correctly. The old software would often no longer function properly.We have designed a new I2C interface, which uses the USB bus instead of the parallel port to connect to the PC, so that it can be easily used with newer PCs and modern operating systems.