How To Build A Flashlight That Runs On Dead Batteries

The dead shall shine again
Four billion alkaline batteries were shipped in the United States in 2010. Reduce that number—reuse your dead batteries in a joule thief flashlight Photograph by Jonathon Kambouris

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.

The circuit boosts the dwindling voltage but delivers it in pulses too rapid to see. As a result, the LED seems to shine constantly, even though it’s really powered less than half the time. Mount the joule thief on a D-size-battery holder, and it makes a handy flashlight, one that gives your old batteries new life.

This article originally appeared in the July 2015 issue of Popular Science, under the title “A Flashlight That Runs on Dead Batteries.”


  • Time: 2 hours
  • Cost: $30
  • Difficulty: Medium


  • Soldering iron
  • Wire strippers
  • Scissors
  • Double-stick foam tape
Joule Thief Schematic
Joule Thief Schematic Sean Michael Ragan



Prepare The Battery Box
Attach The Switch
Solder The Switch
Insert LED
Bend and solder LED leads
Install transistor
Install resistor
Prepare battery box leads
Install battery leads in protoboard
Solder battery leads
Insert wires
Solder toroid wires
Thread wires through toroid
Sew the first toroid wire
Sew the second toroid wire
Thread the toroid wires through the board
Solder toroid wires to board
Test the circuit
Attach foam tape