74LS138 3-to-8 Demux LED Chaser

An eight-LED chaser driven by the textbook 74LS138 3-to-8 line decoder — binary counter on the inputs, one-hot LEDs on the outputs.

Difficulty: Breadboard. Estimated build time: about 35 minutes. Estimated parts cost: about US$2.80. 8-part bill of materials with live supplier pricing. A self-running LED chaser that walks a single lit LED across eight positions, demonstrating the core building block of every memory chip and address decoder ever made: the 1-of-N decoder.