I turned up this cool project write up that reprogrammed a cheap LED voltmeter for use as a general purpose 7 segment display with I2C communication. These voltmeters cost about $3 instead of $10 for an equivalent LED display backpack from Adafruit. NB the author used a Bus Pirate to troubleshoot the bitbanged I2C protocol!
I adapted the project for a different voltmeter using the same STM8S003F3 microcontroller but with a slightly different layout. Blog post here.
Via the forum.