Single-Transistor AM Radio
Tune in to amplitude-modulated broadcasts with one transistor, one diode, and a hand-wound coil — radio at its most beautiful and minimal.
Difficulty: Breadboard. Estimated build time: about 60 minutes. Estimated parts cost: about US$4.80. 10-line bill of materials. Compare supplier offers when available. A working AM broadcast receiver tuned across 530–1600 kHz with one NPN transistor and a Schottky/germanium detector diode.
Wind the antenna coil on a ferrite rod
Take an 8 mm × 60 mm ferrite rod (the kind in old AM transistor radios) and wind 80 turns of 0.5 mm enamelled copper wire along it as a single layer (about 40 mm of rod surface used up). Use a drop of hot glue at each end to lock the wire in place. Scrape the enamel off the two leads with sandpaper — the enamel is invisible insulation. The coil now has about 250 µH inductance, which resonates with a 365 pF variable cap at roughly 525 kHz; with the cap reduced to 30 pF, the resonance climbs to about 1830 kHz, covering the full AM broadcast band.
Build the tuner and detector
Wire one end of the coil to one terminal of the 365 pF variable capacitor. Tie the other end of the coil + the other terminal of the cap to ground (a long alligator-clipped wire to a cold-water pipe is the classic ground; even just a 2 m wire flat on the floor works). The 'live' side of the coil/cap also connects through the 1N5711 Schottky diode (cathode toward output) to one plate of a 220 nF coupling cap. The other plate of the 220 nF cap goes to the transistor's base via a 100 k bias resistor that pulls the base to +9 V.
Wire the amplifier and listen
Put the 2N2222 in common-emitter: emitter to ground, collector to +9 V through an 8 ohm earpiece in series with a 220 ohm resistor (limits standing current to about 40 mA so the transistor doesn't roast). Attach 2 to 4 m of wire flopping across the room as your antenna — clip it to the live side of the tank. At dusk, rotate the variable cap slowly. You'll hear a faint tinny voice swell up, then fade; tune for the loudest peak. Try a longer antenna, a better ground (cold-water pipe!), and the volume jumps noticeably.