One of our Hackaday favorites, [Sprite_tm] made my morning when he sent this in. He built a driver circuit for a HP inkjet cartridge that allows him to print by hand. Ideal for printing on other people, their white boards or their beer. He had to do some blackbox reverse engineering to figure out what the onboard driver chip does on the cartridge. Considering the task, the circuit is surprisingly simple. It has some ATTINY brains, some driver transistors, a data bus and a DC/DC power converter to get the required
1.21 gigawatts, er 20 volts to drive the cartridge.