Introduction: Pocket Flashlight Powered by 1 AA Sized Battery

This pocket flashlight uses only 1 AA sized battery to power up 2X 5mm white LEDs (light emitting diodes). A 1.5V battery doesn't have a high enough voltage to power up those LEDs. We need a circuit to boost the input voltage to the forward voltage of the LEDs to light them up. The circuit in this case is the blocking oscillator (aka joule thief) circuit topology. It's called a joule thief because it's able to light up those LEDs with a used battery until it runs completely flat.

I have made a smaller joule thief circuit using a smaller core in the video above.

Step 1: Gather Required Tools and Components

Required tools:

  1. Soldering iron
  2. Solder
  3. Pliers for cutting component leads
  4. Penknife
  5. Hot glue gun with glue stick
  6. Breadboard (optional)

Required components:

  1. 1Ω resistor, 1/4W (1pc)
  2. Connecting wires (solid core)
  3. 18650 Li-ion cell holder for 2 18650 cells (1pc)
  4. AA battery holder (1pc)
  5. 2N2222 or 2N4401or 2N3094 NPN transistor.
  6. Ready made transformer with 2 set of windings, or a salvaged mini common mode choke, or you can make your own transformer.

Step 2: Making Your Own Transformer

I have found a suitable inductor for my project, but I'll show you how to make a homemade transformer.

For homemade transformer, you will need

  1. A mini toroidal core that you can find from a broken CFL
  2. some enameled copper wires that you can salvage from any electronics that contains a ferrite core transformer. Or you can just buy your own.
  3. You might want to salvage a mini ferrite core transformer from a cell-phone charger and use it to make your own inductor.

Gather 2 strands of enameled copper wires and wind 12 turns of wire around the toroid core as shown. You should get a 1:1 ratio on your transformer. Make sure to not overlap the wires while winding. I used ø0. 5mm enameled copper wires. You can experiment the number of turns and wire gauge for the best results

Step 3: Soldering the Components

Refer the schematic as shown and Solder components on a prototyping board. You might need to cut the board down to size with a hacksaw to fit in the case. Solder down all components first.

Step 4: Solder All the Connections

Use solid core wire to bridge all the connections. Strip off insulation if there's any, you won't need them.

Step 5: Testing

Before putting everything into the case, test your electronics by soldering the battery holder to the circuit which you've just built previously.

Step 6: Make Cutouts on Your Case

I've used a case that fits 2X Li-ion 18650 cells. Cut off any ribs that obstructs the placement of your circuit and battery holder. Cut out holes for the LEDs. Place your assembled circuit and battery holder into the case to see how it looks, do not proceed glue them down. Consider leaving some space for your switch (I used a mini slide switch in this case). Then make cutouts for your switch at the desired location.

Step 7: Putting Everything Together

Solder an additional switch in series to the circuit. Test to see if the switch works. AA sized battery of 0.5V - 1.5V should light up the LEDs. Then proceed to glue everything down. That's it, you've made your own 1.5V flashlight. Thanks for reading my instructable and don't forget to vote.

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