I had a small motorbike whose light got dim when rpm were lower.
in start I tried some big led but as voltage got lower they got exceptionally dim.
so I used lm338(similar to lm317) to regulate voltage(v input was 8-13 volts) lm338 is used to upgrade it to even higher outputs and hence more light.
1.100 ohm resistor 1w=3
5.leds to your need
the zener diode,diode and resistor in last are needed if you are using some cheapo ic
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Step 1: Diodes
I used bridge rectifier to ensure that no reverse polarity will occur as mechanics some times roll over the wires moreover easy installation con be done as these ic's don't like reverse polarity.i used doubled up diodes as these were of smaller size
Step 2: Ic Adjustment
so let r1 be 10 ohm and r2 be 20 ohm by combining two resistors
ZOOM IN THE LOWER IMAGES FOR ARRANGEMENT.
Step 3: Led
add led panel.mine taken out from old portable light
I later on used two one watt leds
Step 4: Cover Up
cover up a little to make safer.