You can, of course, buy I2C ADC chips and modules, but when you’ve already got a microcontroller that has ADC peripherals on board, why bother? [Pawel] wired up a tremendously simple circuit ...

[Raendra] posted a guide for using one of these chips, it’s a Microchip MCP23008 chip that uses the I2C protocol for communications. You are probably already familiar with using shift registers ...