2N3904 BJT Astable Multivibrator

Two transistors, two caps, four resistors — the classic blinker that taught a generation how feedback works, no IC required.

Difficulty: Breadboard. Estimated build time: about 30 minutes. Estimated parts cost: about US$1.80. 6-part bill of materials with live supplier pricing. An astable multivibrator built from two 2N3904 NPN transistors that swap who is conducting roughly twice a second, blinking two LEDs out of phase with each other.