Do-It-Yourself Flashing Rear Bike Light: Part 2

Build a 12 volt rear bike light

disclaimer
posted: 2013-01-09

my second attempt at building a rear bike light

my second attempt

With commercial products being so good these days, why would anyone want to build their own rear bike light? Here’s why: it’s just plain fun, with each project you can learn something new, your design can be as crazy as you desire. If nothing else, it’s a conversation piece for when you’re waiting for that impossibly long traffic light to finally go green.

I run both front and rear lights from a single, 12 volt sealed lead-acid battery. Switches are conveniently located on the headlight pod, along with the charging connector. Why 12 volts? Well, it gives me the option of operating just about any automotive device, such as an air horn, or, with the addition of an inverter, an espresso machine.

My light is housed in a commercial bike light shell, but you could also use an automotive marker, or clearance light, or anything that captures you imagination. The circuit is simple: the output of a 555 timer connects to the base of transistor, which drives 16 LEDs. That’s pretty much it. Frequency is set at a smidge under 3 hertz.

First, some design and building tips. . .

  • Design and test for your needs, e.g. I wanted the LEDs to be sufficiently bright when the battery is low (11 volts), but not burn out if I accidentally turn on the light while the battery charger is connected (about 14.8 volts).
  • Calculate power ratings. Ideally, choose components with approximately twice the power rating required. As an example, the maximum current through the circuit happens if I accidentally turn on the rear light while the charger is connected. In this case, about 3.6 volts is dropped across the 69.1 ohm resistor used to limit the current through a bank of 4 LEDs:

    P = E2 / R, or P = (3.6 x 3.6) / 69.1 = 0.188 watts

    Since this is the worst-case scenario, a ¼ watt resistor is well within spec.
  • As you build the circuit, frequently check for continuity. Inspect your soldering with a magnifying glass. Repair any dubious solder joints and remove solder bridges—I run an X-ACTO blade between the circuit board traces to remove those little oopsies, then clean up the bits of solder with alcohol and a cotton swab. I like to double-check all connections before dropping a chip into the socket and powering up.
  • Use a fuse! I look at a fuse as being there more to protect me than the circuit. A shorted battery is not a pretty thing. Better to blow a fuse than set your cycling shorts ablaze. While the project is still on the breadboard, measure the total current, then select a fast blow fuse with approximately 25% higher current rating than the maximum current of the project.
  • For anything that needs to be cut or drilled, consider creating a template in Illustrator or other page layout program. Use rubber cement to temporarily glue the printed template to the work piece. This is easier than using a ruler and scriber to layout the project. For noncritical work, fine cross hairs on a paper template are accurate enough to mark holes. Use a prick punch, then centre punch before drilling.
  • With the basic design complete, gather all the components. I store everything in a small tin. If I’m feeling especially organized (doubtful) I’ll even label the tin with the project name and date.
  • Breadboard the circuit and thoroughly test it before firing up the soldering iron. Now’s the time to hem and haw, make strange thinking noises, ponder the meaning of life, but most importantly, measure and record voltages/current.

Enough of that noise, how about some pictures. . .

breadboarding the design

breadboarding the design

Testing the circuit on a breadboard (your test circuit will be much neater, won’t it!).

the original light

the original light

The donor light, seems almost a shame not to use it as it is.

removing unneeded parts

removing unneeded parts

Won’t be needing the circuit board and battery terminals. I’ll save these for another project.

circuit board with template

circuit board with template

Paper template made with Adobe Illustrator, rubber-cemented in place.

cutting the circuit board

cutting the circuit board

Cutting the circuit board with a jeweler’s saw. Don’t have one? Get one, they’re great! Get a bunch of extra blades while you are at it ’cause they’re so easy to break.

circuit board cut to shape

circuit board cut to shape

Circuit board cut to shape, cleaned, and ready for components.

all components soldered in place

all components soldered in place

Components soldered in place, yours will be much neater, right?

back of circuit board after soldering

back of circuit board after soldering

Some dubious soldering, you’ll do better, won’t you?

testing the circuit

testing the circuit

Testing the circuit after cleaning up some of the soldering. Everything is working. The LEDs that point outwards are actually on, but hard to see from this angle.

completed light without lens

completed light without lens

After testing for several hours, I assembled the light and re-tested. Parts of the case had to be trimmed to accommodate PCB which now snaps nicely into place. No screws, or glue required.

Velcro glued to back of light

Velcro glued to back of light

Light will be Velcroed to a rack pack. The back is built up with foam which is curved and angled to match the contour of rack pack, then everything is glued with Household Goop adhesive, eclecticproducts.com.

completed light ready to go

completed light ready to go

Flashing rear bike light is complete and ready to go. The fuse is located in a separate fuse box Velcroed to the battery. It contains fuses for the headlight as well.

click image for parts list and schematic (PDF)

click image for parts list and schematic (PDF)

Design Considerations

  • Brightness
    I built my first rear flashing light (Do-It-Yourself Flashing Rear Bike Light: Part 1) because I wanted something brighter than was commercially available at the time. The only way to do this was to cram a gaggle or two of anemic LEDs into a light housing. With the advent of power LEDs, a tail light containing even a single emitter is enough to light your butt sufficiently to mimic a small nuclear event. An unfortunate side effect is that all this brightness is concentrated in a point source that can be so intense it’s distracting for other others sharing the road. Many LEDs come with a warning not to look directly at them, something to consider.
  • Frequency
    Another thing to consider is the flashing frequency. There is some concern about flashing bike lights possibly causing seizures in people with photosensitive epilepsy, wikipedia.org/wiki/Photosensitive_epilepsy. From what I’ve read, frequencies below 3 Hz may be less problematic. I have no expertise in this matter, but it’s the reason I’ve chosen to keep the flashing frequency below 3 Hz.
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