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-part bill of materials with live supplier pricing. 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.