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.

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!

To start, log into “My Mouser” and go to your order history (not invoice history). Click on an old order, and after it opens up, click on “print view”. Print view strips out the form elements that just make things more complicated. Now go to File, Save As and save the web page as a compiled page (MHT format). Sorry firefox, I do love thee as my browser of choice, but this only works in IE as far as I know.

Now open your mht file in excel, you should see that excel properly parsed the order table into neat little rows and columns. Now, open the excel file I’ve provided at the end of this post. Right click on the worksheet tab and choose “Move or copy…”. Copy the worksheet to the mouser workbook. After you have the two worksheets together, rename the mouser worksheet into “Web”. You’re almost done!

At the top of my worksheet there are two cell fields, starting row number and starting cell reference. The row number is where the parts table starts, it has been row 31 on all my orders so far. Starting cell reference is the worksheet reference for the first cell containing parts data, this has been B31 for all my orders so far. Be sure to include the sheet reference as well, so you want that to read “Web!B31″. If your order history is like mine, you may not need to change either of these. Once you get it right, you’ll see the rest of the spreadsheet fill in with your parts data. Now you can save this as an excel file, or export it as CSV, or whatever.

Download the excel spreadsheet

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.

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!