justDIY Project Log

As the (normal) bicycling season draws to a close for my latitude, I’m nearing completion on my bicycle taillight project. I sent out a few weeks ago for some professionally fabricated pcbs and as usual, they look great. There were a few bugs that were entirely my fault, but nothing serious enough to stop the pcb from doing its job.

I added two “features” to the pcb before sending the design out, both of which were untested in the initial prototypes. The main feature I wanted to have was “motion detection”, so the taillight would shut down should the bike become idle for a period of time. No need to be wasting precious photons while the bike is leaned up against a tree or parked in a bike rack. Motion detection is provided by a roller ball switch, intended to replace the old fashioned mercury switch. A tiny gold plated ball rolls around in a plastic and metal cage, completing electrical circuits during its travel. The light’s micro-controller recognizes these impulses and continues to let the light function. As soon as the tilt switch stops changing state, resting as either a short or open circuit, the uC begins a count. When that count totals some arbitrary number, the light returns to a standby mode with the SMPS in shutdown. The uC then watches the tilt sensor for the state to change again and upon a change, resumes the previous operational mode.

The second feature is a battery minder circuit. Using a 2.5v precision reference, the micro-controller samples the battery voltage using its on board ADC. The idea is to detect a weak battery condition and operate the SMPS at a lower duty cycle, to make the most of the remaining power. The assumption here is that some light is better than no light in terms of safety. One of my pcb bugs lies in this circuit. The Microchip 12F683 uC I selected for this project is an 8 pin device. Its voltage reference pin is also multiplexed with the programming clock. In my design, I had made the error of connecting the vref pin directly to the voltage reference output, which is biased with a 1k resistor to the Vdd rail (bat +). So effectively, I have a very strong pull-up to 2.5v on that pin. This made programming the PIC impossible as it could not detect clock transitions. I will try salvaging these PCBs by changing to a 10k or 20k bias resistor on the reference, or cutting the trace leading to the Vref pin and soldering a 10k+ resistor in series, since we don’t need any current on that pin, just voltage.

Once I polish the code a bit more, I’ll be looking for a few folks to send a sample units to in exchange for reviews and feedback, down the road I would like to sell these either as a kit or a pre-assembled unit.

Popularity: 9% [?]

I just received these FedEX on Tuesday, fresh from China via Colorado.

The first stack of boards is the prototype taillight driver, sporting a tilt switch for motion detection. The second board is a pretty similar design, with a bigger inductor and more compact layout. The intention here is to run a trio of Lumiled’s Rebel leds at 0.5 to 1w each off 4AA batteries, for a compact self contained headlight. More details on that idea later!

Popularity: 8% [?]

A little quick work with the rattle can this weekend “finished off” my revision 2 and revision 3 bike taillights. Revision three is nearly the ‘final product’ but still lacking some automatic control circuitry that I want to implement, and a few tweaks to the firmware to make it simpler to use.

I haven’t mastered cutting a straight line with the dremel yet, once the cutting wheel bites into that thin steel it goes the direction it wants to go!

I also painted the circuit boards, masking off each LED lens on the 2×8 array so they’d stay nice and bright. I also masked the smt button, the switcher and the contact springs on the battery clips. I probably didn’t need to mask the switcher – I wasn’t sure what the paint would do it it, seeing as how it’s handling quite a bit of power at a high frequency.

I’ve also posted some new videos to my youtube channel – nothing too exciting. There’s a naked revision 3 doing its thing and a side by side of 2 and 3 post paint job.

Popularity: 7% [?]

I completed PCB revision three of the mint tin bike light on Tuesday, but due to lack of batteries for the camera, no pictures were taken! Luckily I’ve found and recharged a second set of batteries and the camera is once again operational.

Feature-wise, this revision adds nothing over the previous light, all the changes are in board design. Firstly, the artwork was redone using polygon pours instead of straight point to point wiring (traces). The revision two switcher was running pretty warm, mostly because it didn’t have much copper to dump the heat into.

The switcher’s ground pin is now tied directly into a very large copper pour, as are the Vin and Switch pins. Using a burning finger temperature probe, the chip remained at or below T_body even operating in constant on mode at full power. Compared to the revision two board which saw the switcher running quite hot in constant on mode.

The current sensing resistor was moved a lot closer to the feedback pin. With a feedback voltage of 190mV, the tiny resistance of the trace was actually affecting output. Shortening the trace to roughly 1mm has helped a great deal.

Finally, the layout for the battery clips was fixed, and generous polygon pours were drawn around the pads. The spring clips are now soldered down very firmly and hold the batteries quite well. I have yet to take this unit on the trail, so we’ll see if a rubber band is required or not to retain the batteries while bouncing along.

I plan on making one more prototype before sending the design off to Custom PCB or Gold Phoenix. I think I’ll eliminate the battery clips on the chance excessive force could cause one to separate from the laminate and severely damage the pcb. I also want to try a board that hosts both driver circuit and LEDs. Additionally, I plan to add a tilt / vibration sensing switch (roller ball switch), so inactivity of the bike can be detected and the light switched off to save on batteries.

Thanks for reading!

Popularity: 10% [?]

I finished my first snapled array, and they are damn impressive! I was expecting slightly better performance than the superflux, but was blown away. I haven’t come up with a method of comparing the two yet, as I don’t have a light meter. A side by side with the reflect signs outside the house would be nice, but it’s raining cats and dogs right now!

Here’s a size comparison of the snapled versus a 3mm superflux led. Both have a body measuring 7.6mm, but the snapled has those huge contacts, and a much heavier internal structure compared to the superflux. The 5mm snapled lens also looks huge compared to the superflux.

Hand soldering the snapled smt style is fairly easy – this connection lifted up on me because I was pressing down on the opposite side. Soldering the rest I just placed the led and then slid the iron in next to the contact without touching it, then fed in the solder, which sucked the contact right onto the pad like it’s supposed to.

The finished product, before washing. Once I decide on a layout I like, I’ll probably have some boards made and will try reflowing these either in a toaster over or on a skillet.

Popularity: 4% [?]