2N3904 NPN General-Purpose Transistor, TO-92 (3-Piece Pack)

Alongside the BC547 and 2N2222, the 2N3904 is one of the handful of small-signal NPN transistors that show up constantly across published circuit designs, kits, and project schematics, which makes it worth stocking even though its role largely overlaps with its better-known siblings. Its slightly different gain and frequency response compared to the BC547 means some designs specifically call out the 2N3904 by part number, so having it on hand avoids substitution guesswork when following a schematic exactly as published. Like its counterparts, it handles small-load switching and basic amplification duties in low-power analog and digital circuits.