Lantern LED retrofit
May 2012
About
A friend had purchased some battery powered outdoor lanterns, which he really liked and came in useful a lot, but had the unfortunate ability to drain the two AAA batteries really quickly. He suggested replacing the incandescent bulb with an LED — which sounds reasonable enough — but bright, white LEDs usually need about 3V to work, and two rechargeable AAAs won't provide that.
I was familiar with the joule thief circuit, but these will run a battery down to a very low voltage, which is fine if you're just using old alkaline batteries, but not so great if you want rechargeable batteries you can reuse.
I got around this by putting two diodes in series with the batteries. The diodes have a forward voltage drop of about 0.75V, meaning that the diode won't conduct if the voltage drops below 0.75V, providing a safety cutoff.
Schematic
My design is simply an extension of the venerable joule thief, tailored to my specific application. I incorporated the pre-existing power switch and battery pack from the lantern, but changed the function of the switch to select between one or two cells — giving the user the flexibility of using a single alkaline cell if desired.
Components
- 1x 60uH dual inductor (~$0.75)
- 2x 1N4004 Diode (~$0.05)
- 1x BC548 NPN Bipolar Transistor (~$0.05)
- 1x 470 Ohm Resistor (~$0.01)
Notes
Further work could include:
- Implementing US Patent 4,734,658 which utilises a JFET rather than a BJT transistor to decrease operating voltage down to 0.1V.
- Developing a method for voltage cutoff which does not reduce voltage presented to joule thief.