errite toroid coreexternal image ir?t=joulethief2491-20&l=as2&o=1&a=B008LTJBAW (can also be found in old CFL bulbs)
– Old/Used/Homemade Batteries (can also be found in garbage cans or old remote controls you haven’t used in years)
2N3904 NPN transistorexternal image ir?t=joulethief2491-20&l=as2&o=1&a=B007CMFP5E (almost every kind of electronics has these but the small things like cell phones have teensy ones that you probably won’t want to use)
1k Ohm resistorexternal image ir?t=joulethief2491-20&l=as2&o=1&a=B00B5R8950 (the size isn’t all that important but will affect the circuit.
LEDexternal image ir?t=joulethief2491-20&l=as2&o=1&a=B007HKWLXU (by itself or inside a flashlight)
Multimeterexternal image ir?t=joulethief2491-20&l=as2&o=1&a=B005EK3NRS (optional but it’ll help troubleshoot the circuit)
22 gauge magnet wireexternal image ir?t=joulethief2491-20&l=as2&o=1&a=B00BJMVMQE
AA battery holderexternal image ir?t=joulethief2491-20&l=as2&o=1&a=B008SNZQ0A

Don’t throw out that seemingly lifeless battery—it’s not dead yet. A brand-new alkaline battery cell has an electric potential of about 1.5 volts, which drops as the juice runs out. The voltage eventually becomes too low to power most devices, but there’s still energy trapped inside the battery—as much as 15 percent of the original charge. By wiring a circuit called a “joule thief,” you can tap the last of that power to light a white LED.
GOOGLE “how to build a joule thief” for different ways to make the device.