I'm just wonder. I have seen several trailer LED lamps that fits to both 12V and 24V circuits. How can i make a circuit like that?
Such circuits have a current stabilizer.
You could make your circuit with a dpdt switch or a couple of jumper links. This would involve splitting your LEDs into 2 groups, electrically speaking. The switch/jumper links would allow the two groups to be connected in series for 24V or parallel for 12V. This would not be foolproof, if connected to 24V when set for 12V, it would be damaged.
Specifically, a constant-current driver.
Preferably, a switchmode converter which is efficient and does not get significantly hot. Many such modules are available on Aliexpress and the like.
Get lazy, use a buck converter and it will work great!
Constant current LED drivers (the switching ones) are buck converters.
They just have current feedback instead of voltage feedback.
Leo..
I don't think i understand.
Look at this tail light:
It can be powered with 10-30V.
Is there just a kind off buck converter innside these lamps that increases the voltage down to 3V for lighting up the LED's innside?
We cannot see any of the internals for that light, but I suspect it simply has current-limiting transistors which absorb the extra voltage above 12 V (and heat up as a result).
The LEDs are in series groups of 3 or 4 (depending on colour) so they have a combined voltage drop of 9 or 10 V, not 3.
Yes, of course. That sounds logic for me 
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