Joule Thief — LED from a Dead AA
One transistor, one toroid, one resistor — light a 3 V LED from a 'dead' 0.9 V battery by stealing energy in microsecond bursts.
Difficulty: Through-hole. Estimated build time: about 40 minutes. Estimated parts cost: about US$3.20. 4-line bill of materials. Compare supplier offers when available. A self-oscillating boost converter that lights a white LED from a single AA battery — including a so-called 'dead' AA that has dropped to 0.9 V and would not light an LED directly.
Wind the transformer
Take a small ferrite toroid (FT-37-43 or any equivalent) and a foot of thin enamelled wire. Fold the wire in half so you have a doubled length, then wind 8 to 10 bifilar turns through the toroid (both wires together, evenly spaced around the core). Cut the loop at the fold — you now have four wire ends. Identify the two wires that form one continuous winding, and the two that form the other. Use a multimeter on continuity to be sure.
Phase the windings
This is the only step that ever goes wrong. Connect the START of winding A and the END of winding B together — that joint is your transistor's collector node. The OTHER end of winding A is your battery positive terminal. The OTHER end of winding B (through a 1 kΩ resistor) is your transistor's base. If you wire it backwards, the circuit just sits there silently. Swap the two ends of one winding and try again.
Wire the transistor
2N3904 in TO-92, flat face toward you, pins (left to right): emitter, base, collector. Emitter → battery negative. Base → 1 kΩ → second winding (as above). Collector → joint of the two windings.
Add the LED
Connect a white 5 mm LED from the collector node (anode) to battery negative (cathode). When the toroid flies back, the LED sees a brief high-voltage pulse that drives current through it. With a fresh AA the LED is bright; with a 'dead' 0.8 V AA it is dim but visible — the joule thief is happily extracting the last bit of usable energy.
Probe the waveforms (optional)
Hook a scope to the collector and set 5 V/div, 5 µs/div. You should see a clean ringing waveform at 30–80 kHz, with brief 5–8 V spikes at each switch-off. The LED's instantaneous current during those spikes is huge; its average is small. This is exactly how an LED driver IC works, just with discrete parts you can probe.