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-line bill of materials. Compare supplier offers when available. 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.
Place the transistors and the symmetric load
Drop two 2N3904s into the breadboard with their flat faces visible. Each gets a 1 kΩ resistor from collector to +5 V (the load), and an LED in series with a 470 Ω resistor from collector to GND so you can watch the collector swing. The two halves are mirror images of each other — that symmetry is what makes the circuit oscillate.
Wire the base bias resistors
From +5 V to each transistor's base, drop a 47 kΩ resistor. This is what eventually pulls the off-side transistor back into conduction after the cap pulls it down. Bigger R = slower blink; smaller R = faster but eventually the transistors stay on permanently. 47 kΩ is the sweet spot.
Cross-couple the timing caps
From transistor A's collector to transistor B's base, place a 10 µF electrolytic (positive lead at the collector — it spends more time at +5 V). Same in the other direction: transistor B's collector to transistor A's base, another 10 µF. These caps are the memory of the circuit; their charge state determines who is conducting right now.
Power up and watch
Apply 5 V. The two LEDs blink alternately at about 1.5 Hz. If both LEDs come on solid, the circuit got stuck in a metastable state — touch one base briefly to ground to kick it into oscillation. Once it starts, it self-sustains forever.
Tune the frequency
Replace the 10 µF caps with 1 µF and the blink jumps to 15 Hz. Replace with 100 nF and you cross into the audio range — connect an 8 Ω speaker through a 100 µF series cap from one collector to GND and you hear a 1.5 kHz tone. This is the same conversion every astable circuit lets you make: bigger C = more time = lower frequency.