Bicycle Headlight With Joule Thief LED Circuit

This circuit idea is started for best utilizing 1.5V batteries (AA, AAA) thrown away after use.

I knew still significant energy is available in the used batteries even the home appliances are declaring they are fully drained.

One type of circuit that can pull out all remaining juice of energy from the used battery is Joule Thief.

Most home appliances such as an Electronic door lock or remote controller declares that the battery is fully used when its voltage output becomes less than 1.3V.

But the Joule thief circuit can pull out any available energy until the battery’s internal resistance becomes unable to create any useful output voltage such as 1V or less than that.

As diverse web page information and circuit operational schemes explaining videos are abundant on Internet, I will not explain the concept of Joule thief circuit operation.

As shown in the picture above, a used battery (AA, AAA size) can still shine your road ahead while riding a bicycle at night hours.

This is a homemade headlight to be attached to the bicycle handle.

As cardboard type of thick paper is used for making headlight body and lamp holder, the overall appearance is not flashy.

Also, the headlight size becomes a little bit bigger for mounting on the bicycle handle.

Maybe the next version can have a more refined appearance when utilizing new materials and applying design improvement concepts attained by making this headlight prototype.

Anyway, it serves well as a bicycle headlight to light the road ahead during night hours as shown in the picture above.

Let's look more details at the subcomponents of the headlight.

The headlight consists of 4 major parts as shown in the picture above.

- Lamp part includes 4 white LEDs and paper LED housing

- Joule thief circuit board

- AAA battery and battery holder

- Toggle switch and acrylic bottom cover

When utilizing a 3D printer, a more compact, refined, and sturdy headlight body case can be produced.

But I don't have such luxury equipment for producing customized equipment enclosures.

Anyway, the used thick paper board still can support easy cutting, bonding, and shaping except for making a strong body case.

Let's look at each subcomponent how it was made.

I used a thrown-away biscuit box as the headlight body for housing the circuit and battery.

The lamp holder cone is made by compass and pencil and attached to the front panel with glue as shown in the picture above

As the paper headlight body is too soft and frail, an internal supporter structure is inserted inside the body.

But the bottom cover is made with the acrylic board as a toggle switch should be attached to the cover.

Strong adhesive is used for mounting the toggle switch to the acrylic cover.

For mounting the headlight body on the bicycle handle, an additional acrylic board is attached to the bottom of the headlight body. (I'll show you later the smartphone holder accessory which actually mounts and holds the headlight body)

I think this circuit is the most useful one among the analog circuits I know.

As I told you before, I will not explain the operation details of this circuit.

Instead, I'll focus on how this circuit is made as shown in the schematics diagram above.

The most important part of this circuit is the small transformer made with a ferrite core and twisted wires.

I prefer to use twisted wire instead of enamel wire as finding material is very easy.

You can utilize any pair of twisted wires from the unused LAN cable.

When you strip the sheath of any LAN cable, you can get 4 pairs of twisted wires.

About 20cm length of two wires (Pair of wires) is enough for making the small transformer.

I salvaged ferrite core from a broke down PC power supply circuit.

Or you can buy about 3~4cm diameter of ferrite core from an Internet online shop.

When wires and core are ready, just wind cable 20 times around core as shown in the schematics above.

You can use any general-purpose transistors such as 2N3904, 2N2222, 2N4401, BC547, and so on.

Although I used 2K VR, you can use a 1K 1/4W resistor.

But for driving more than 1 LED, VR will be a good choice to adjust resistance value according to the circuit operation requirement.

Careful soldering 4 wires of the mini transformer as shown in the schematics above is the most important for the correct operation of the circuit.

When everything is assembled correctly, a fully functional headlight can be made.

According to my experience, a single used AAA battery can shine your road for roughly more than 6 hours.

After that brightness becomes not suitable for bicycle headlight purposes.

Thank you for reading this story.