PCF8574 I²C 8-LED Bargraph

Eight LEDs from two wires — an I²C GPIO expander in 30 minutes.

Difficulty: Breadboard. Estimated build time: about 35 minutes. Estimated parts cost: about US$2.50. 6-part bill of materials with live supplier pricing. The PCF8574 is a slow but useful I²C-to-parallel chip: two wires in (SDA + SCL), eight quasi-bidirectional GPIOs out.