Introduction
Our family takes an annual camping holiday to the Isles of Scilly. My normal torch (a Maglight AA) usually ends up flat after a few days. This year I decided to build myself an LED torch.
Instead of building the torch body from scratch I converted a cheap D cell model.
The Circuit
I used a Luxeon K2 LED because of its immense light output and the fact that it is
relatively easy to solder. A current between 500mA and 1200mA with a forward drop
of 3.8 volts appears to be ideal for driving it.
I initially decided to use a simple constant current regulator based on a design on the Instructables website. When powered from the three D cells the voltage drop through the MOSFET was too high and the LED only partially lit.
I looked around for an integrated LED boost driver with an input voltage of less than 4.5v and current limiting up to 1A but there appears not to be any! None that I could order from Farnell anyway. The CAT4139 and CAT4240 come close with a current handling of 750mA but Farnell do not stock them.
In the end I decided on using a boost converter and then the current regulator in series. I looked around and found an IC hidden away in the Anolog Devices site: the ADP1111. It's a canned boost converter in a handy DIP8 package—just add an inductor and Schottky diode. It comes in a variety of different output powers including an adjustable one. I chose the highest output: 12 volts.
The drawback is that the ADP1111 requires a beefy 9 watt inductor to work. I chose a Schottky diode with a 3A 30v rating to ensure adequate peak current handling.
Assembly
The original torch head used a piece of circular aluminium to connect to the switch on
the inside of the torch body and a metal post to connect to the positive "bump" on the
batteries. I mimicked this with a circular section of 4mm aluminium which also served
as a heatsink to the MOSFET and the LED. The led and MOSFET where both stuck to the
heatsink with some
adhesive heatsink tape.
To connect to the (negative) aluminium piece I soldered to a nut and bolted it through
a hole.
Both parts of the circuit are assembled on strip board—I did not have time to create PCBs even if I wanted to. Everything mounted to the aluminium (circuit boards, reflector and positive terminal) is held with epoxy resin.
The boost converter is the circuit hanging down from the aluminium in the picture to the right. Note its relatively large diode and inductor above it. The current regulator is the circuit to the left of the Lego. The Lego has a metal conductor glued to the top and is used to connect to the positive battery terminal. The bolt for electrically connecting to the aluminium is visible in the center of the image.
Conclusion
The LED driver circuit works exactly how it was designed. The beam remains at a constant
brightness right down to the point when the batteries are exhausted. If I ever come back
to the project I would be tempted to scrap the entire circuit and instead do the boost
conversion and current regulation in one with a microcontroller eliminating the wasteful
situation where the MOSFET is in its linear region.
The torch's reflector is not suited to an LED—a fair ammount of light leaks sideways. Unfortunately there is nothing much that can be done about the problem aside from moulding a new lens. The beam is powerful enough for this not to be a big problem, however.
The power allowed through the regulator can be adjusted by changing the value of the sense resistor (see the original article for more on this). I left it at 1 ohm giving a 500mA drive current. I looked up a few alkaline battery datasheets and most suggest about 10 hours life with this kind of drain, dropping to a mere 1 hour at 1000mA.