Contant Current Regulator
Advanced modifications:
I have tried to keep this design as simple and as easy to assemble as possible, whist allowing it to run on as many different batteries types. The design is incompatible with a two cell batteries such as the Speleo Technics FX2, as the LED voltage is higher than the battery voltage (2.4V). To use an FX2 battery a step-up switching regulator would be required.
Although the circuit is capable of running on voltages much higher the 6V, it is not recommended, the use of a step-down switching regulator would increase efficiency, giving a much longer battery life.
For Speleo Technics FX3 and Headlight batteries the LED voltage is very similar to the battery voltage, to optimise these designs a Buck-Boost switching regulator is required. These regulators step-down the voltage when the battery is fully charged, as the battery discharges and the voltage drops they are able to change over and step-up the voltage.
For the design proposed here using 4x AA batteries we could use a step-down switching regulator, however most of the off the shelf designs require an input above 6V and hence are unsuitable. To build a switching regulator from scratch is beyond the scope of this article. A linear current regulator can be used as an alternative, and is recommended for those with good soldering skills. The current regulator will ensure a constant LED current and hence LED brightness, up until the point where the battery voltage drops below ~3.8V. (The light output will be regulated for more than 95% of the battery life when using NiMH cells.) When the battery voltage drops below this level the light output will gracefully diminish. The user will never be plunged in to total darkness.
As this is a linear regulator its efficiency is determined by the battery voltage, when using 4x NiMH cells the efficiency will be close to that of a switching regulator.
Current Regulator Schematic and Design Equations
Both the transistor and current regulator are available from RS and Farnell, pricing information and datasheets can be found on their websites.