It all depends on the input (battery/batteries) and your desired brightness vs. runtime. The fb1 holds a single 123, assuming you will use a R123, you will most likely need a buck/boost converter like the GD designed for single cell lithium ions or a buck coverter (SOB) depending on the forward voltage of the led (voltage in needs to be greater than forward voltage of the led). Once the type of converter is decided, you need to decide on the current. The higher the current, the brighter the led, the shorter the runtime. I like boards around 500-650ma for the newer led's.
The shoppe's converters are labeled as the type and then the current. A GD750 would be a buck/boost type regulated for 750mA. A SOB750 would be a buck converter at 750ma.
Hope this helps.
Chris
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