LM317 Constant-Current LED Driver

Drive a string of LEDs at a precise, regulated current with just an LM317 and one resistor — the simplest two-component constant-current source in the analog toolkit.

Difficulty: Breadboard. Estimated build time: about 20 minutes. Estimated parts cost: about US$2.80. 5-part bill of materials with live supplier pricing. A constant-current source for LEDs that keeps the drive current rock-steady regardless of how many LEDs you put in series or how the supply voltage drifts.