Powering LEDs for motorcycle

i'm going make custom led brake light friends motorcycle. got bunch of red-orange superflux 70ma leds , tomorrow working on layout them. wondering how efficiently power leds controlled either attiny85 or atmega328 depending on end trying them. i'm thinking have 20 30 leds possibly more or less.

powering them 5v regulator seems bad idea if it's linear regulator, lots of wasted energy. switchmode regulator? maybe better idea although have parts make few different regulators havn't tried yet.

a better idea seems me power them 12v , put every 2 or 3 in series. again, don't know layout think fine. doesn't seem idea power leds directly off 12v line because of flicker. again lights on motorcycle don't seem flicker maybe fine? maybe use regulator doesn't drop voltage much?

any advice on matter nice.

thanks.

i use 12v supply power them - place several in series , add current limiting resistor - more leds add more strings of leds (plus resistor) in parallel first string.  can quite efficient since of battery voltage (and hence power) go leds.

however quick word of caution - need make sure light when battery low (brake lights important!), should assume battery voltage 11v or in calculations (and recalculate 13.8v - battery being fast-charged).

so step 1 lookup forward voltage of leds on datasheet, preferably graph of forward voltage against temperature.  vehicles exposed wide range of temperatures , forward voltage vary quite bit.  wise measure forward voltage @ 70ma - datasheet might "optimistic".

so given maximum forward voltage , minimum battery voltage can work out number of leds in 1 string.  example i'll assume forward voltage of 1.9v , min. battery voltage of 11v - suggests string length of 5 leds (taking max of 9.5v).

then need max current leds, lets 70ma quote.  assume battery voltage can go 13.8v , led forward voltage minimum 1.7v (off top of head, don't trust these values!), string take 8.5v , difference battery string 5.3v.  series resistor 5.3v/0.07a  = 75 ohm.  dissipate 0.4w.

with 30 leds thats 6 strings of 5, ie total current of 6 x 0.07a = 0.42a

one mosfet happily switch this, long logic level 1 - source ground, gate via 220ohm resistor arduino pin (or whatever), drain cathode end of strings.  anode end of strings +12v (don't forget 75 ohm resistor in each string.

finally can calculate min current (assume vbatt = 11v, vforward=1.9v, leaves 1.5v across resistors , 20ma each string instead of 70ma

an alternative scheme use constant current led driver chip - prevent variation of current temperature , battery voltage.

[if used 5v regulator you'd limited 2 leds per string, 15 strings = 1.05a.  regulator need dissipate upto 10w]

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