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 that replacing the incandescent bulb with an LED to me, which sounds reasonable enough - but bright, white LEDs usually need about 3V to work, and two rechargable 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 to power the thing, but not so great if you want to have batteries that can be used over and over again.
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 they so desired.
Components
Prices in AUD, at time of writing
- 1x 60uH dual inductor (element14 $0.75)
- 2x 1N4004 Diode (anywhere ~$0.05)
- 1x BC548 NPN Bipolar Transistor (anywhere ~$0.05)
- 1x 470 Ohm Resistor (anywhere ~$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