Motivation

Find it so weird to recharge almost every day a dedicated battery for the rear light of my cargo e-bike while a there is a huge battery onboard available on this bike to power the motor!

Power leds (3W) became so cheap and come with optics, it is perfect for any project needing efficient light system.

Circuit design

The available power from Brose motor is 14.6 W whose 3.6 W already used by a manufactured front lamp installed on the bike -> 11 W available for this project

Schematic

See constant current instructable led driver circuit : Cheap and efficient, could work with input voltage up to 60 V. Low drop out voltage between input and output so it can drive two led in series from 6 volt input.

The value of R_sense limit the current through the led to 1A.

Dimming the led is achieved with pwm from Attiny85.

Rear light

The rear light has two modes:

1. Constant low power
2. Blink: alternate low power / high power light (best approach for visibility purpose)

Constant low power: pwm ratio is 10/255 -> about 0,25W

Blink: pwm ratio is 10/255 during 500 ms and 60/255 during 50 ms. -> about 0,35 W

Then about 10W still available for the front light (more than enough!)

Front light

The front light has two modes: OFF / ON (pwm dimmed to every available amps)

PCB / stripboard

Components are soldered on a stripboard. it was intended to be a prototype but became the final circuit because works well and perfectly fits in the box.

Rear Led housing

Cree Led XP-E2 are mounted on a 20 mm diameter pcb. 3/4" copper pipe has exactly enough room to hold it. 

Led pcb is glued inside the tube. The tube is filled with waterproof glue and mounted on a twisted aluminum plate.

Attiny85 code

The code is available on github repository ebike_led_driver.