I'm building an improved taillight for my motorcycle. Instead of
an incandescent bulb and reflector, I'm using 80 high-brightness red
LEDs. This offers four advantages:
Instant-on. There is no filament to heat up, so the light comes
on immediately. This should attract others drivers' attention.
Uniform brightness. Most tail lights are brighter in one are than
others. By using 80 LEDs, I can fill the taillight evenly with light,
thereby making the most of the limited area I've got. On cars, this
isn't as much of a problem because they have much larger taillights.
Higher brightness. I can make the brake light brighter than
stock.
But don't worry... I don't intend to blind people. A lot more on this
later.
Lower power. At maximum brightness (which I won't use), this
light takes 20 Watts -- less than the 35 Watts the normal taillight
takes.
The Bike
I don't have any cute kid pictures to show off, so until
then, I've just got
a picture of my bike instead. It's a Ducati 999 in front of the
Rocky Mountains... about a block from my house:
Initial Evaluation
I used a professional light meter from my office to compare
three
vehicles -- my car, the stock bike brake light, and the new brake
light. I tried to find the legal requirements, but it seems that
the law states vehicles must follow the SAE (Society of Automotive
Engineers) standards -- and it costs $200 to get this standard. I think
that all laws should be free to read (and freely distributed), so any
help in getting this
standard
would be appreciated. I noticed that most cars are fairly close in
brightness, so I just used my car as a reference -- that's not perfect
because
I don't
know exactly how it should be measured.
I made four measurements for each condition: Each light had a bright
spot and darker spots, and appeared differently when viewed directly
from behind (on-axis) versus from an angle (off-axis).
Some observations:
First, notice the scale: it's logarithmic so that I can fit
everything on one graph.
Uniformity:
There is a big difference between the bright and dark portions of
the stock Ducati light -- the bulb area is about 50x brighter than the
surrounding area. That's a shame because the surrounding area is so dim
it might as well not be used.
The car's light is more uniform than the bike - the brightest
portions
are only 1.7x
(on-axis) to 8x (off-axis) brighter.
My LED taillight is even more uniform (3.5x to 1.7x brighter)
than either of the the
other lights. I purposefully made the on-axis maximum extra bright by
using a few narrow-beam LEDs -- I'm most worried about the person
directly behind me.
Brightness:
The bike's bright area is pretty comprable to the car.
My LED taillight is capable of more than 10x the brightness of
the car! (but, remember the car probably has 5x the surface area of the
bike's light, so it's not a fair comparison)
Comparison
Cameras don't realy have the dynamic range to compare the
brightnesses of these lights, but the pictures are still fun. You can
tell that the illumination is more even than the bike, but in real life
the bike has a much more pronouced "hot spot" in the center.
0.3 Amps - About evenly matched
(brake lights)
1.4 Amps - Maximum output.
Technical Details
The LEDs I used are the Superflux
LEDs by Lumileds (now Philips). The specific part numbers are
HPWT-BH00-F4000 and HPWT-MH00-G4000, and the run about $0.45 each.
I haven't posed a schematic yet, but it's fairly simple. I still have
not picked the resister values to set the brightnesses -- I'll probably
do that this winter.
Future Design Goals
I want to incorporate a light sensor in to the design. This
would allow for brighter-than-usual operation during the day (so I can
be more visible), but normal operation at night (so I don't blind
people). I've also got an idea for a microcontroller to do some
extra stuff...
My next measurements will be:
- a side-by-side comparison with my car to make sure I've got a
comfortable night-time brightness. (done, above)
- a stand-alone daylight test to see how much brighter I can make it on
sunny days, where I intend to do most of my riding. (not done yet)
