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!

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!

This year I’ve been going on a lot of bike rides with friends, sometimes on public roadways, sometimes after dark. My bike has a nine watt 500 some odd lumen headlight, which makes it easy to see where I’m going, and definitely makes me visible head on. The tail of my bike however still has the stock reflector, plus the little reflector stripes in my shoes, not exactly high visibility. Not wanting to pale in comparison to the headlight, the taillight is a three watt 140 lumen beast powered by three AA rechargeable batteries.

The light is based on a boost converter from National Semiconductor, the LM3410. I’m using the 525kHz SOT-23 version, the LM3410Y. Originally I had trouble with the chip self destructing, as discussed on the Linear1 forums. It was hypothesized either the inductor was underrated or the diode was too slow. Ordering parts for another project later in 2008, I bought some better inductors and diodes, which more closely resembled the specs of parts used in National’s web bench simulator. So, lacking sufficient rear light, I rekindled this project and have a “working prototype” that’s gone on two rides with me so far.

The basic function is fairly simple. The 3410 is a constant current boost (step-up) driver. A small inductor is used to ramp up the input voltage, from 3.6vdc nominal to 15.4v at approximately 200mA. The current is monitored by a one ohm resistor. A pair of output capacitors help smooth out the ripple and an input capacitor helps the batteries cope with the high demand current (as high as 1.5a in some cases). I’m using nickle metal hydride batteries, which have a rather low internal resistance – they’re designed for high demand applications and when fresh, barely sag at all under the load.

Originally I had planned on carrying the batteries directly on the PCB, using some through-hole spring clip battery holders I found in the Sparkfun library. However, AA batteries must be bigger in Colorado than they are in Michigan, because using Sparkfun’s layout gave me about a quarter inch gap between the spring and the battery. The pads were also woefully undersized for physically mounting the clip and holding it securely enough to survive the stress of batter insertion and extraction. So I dropped their layout and drew my own that looks exactly like it, but is based on measurements from a real AA battery.

Along for the ride is a Microchip PIC microcontroller, the 12F683. It provides a bit of user interface for the light, creating different blink patterns as well as putting the light into a “stand by” mode, with the switcher shut down. I’ve programmed several blinking patterns, and somewhat organized them into “modes” which I can select using the little button.

A year ago, I didn’t have any sort of enclosure in mind. The led array was assembled on a ‘standard’ sized protoboard, so I probably thought about using a plastic or aluminum prototype enclosure. However, this year, I was thinking it would be a nice fit for a large mint tin. After printing out some mock-ups and messing around with battery configurations, I settled on using three batteries and having the electronics crammed into one side of the tin with the led array mounted in the lid of the tin. This setup might have worked, except for the battery snafu. I’m using a plastic three cell holder right now, and the extra thickness it adds is preventing the lid from completely closing. It closes enough that the light is easily held shut by some big rubberbands, and it survived bouncing around under my seat for two short rides. The next revision will have the battery situation resolved and I might have a better mounting solution by then too.

Overall I’m very pleased with the outcome of this project. I have more parts on order to make a few more lights for my other bikes and friends, and I want to experiment with other array configurations and colors. There are a two videos of the light on my youtube channel, but they’re nothing to get excited about.

Thanks for reading!

I’ve been trying to build a switcher to provide a portable power source for PDA’s, cell-phones, etc. Not just enough to trickle charge said gizmo for hours, but to charge it as fast as possible, like the cradle or wall plug would do. This requires quite a bit of power. Packaging portable power is proving very tricky. There’s two main design goals I’m trying to meet. My primary goal the past week has been recharging the power-source, and the easiest way to do this is parallel cells. There’s oodles of charge management chips out there designed to handle charging of single lithium-ion cells (or parallel cells). The complexity knob gets turned WAY up once you start talking series cells. So rather than spin my wheels on this problem, I chose to move forward with goal number two. My secondary design goal is to get the power out. Putting the cells in series opens a wide door for easy to use switchmode converters and controllers. I’ve got a bin full of samples from all the big names, and I’m close to settling on a chip. First to prototype is the TPS5430 from Texas Instruments. This chip claims to have a three-amp switch on board, and it’s pretty easy to use. The switcher is internally compensated, eliminating an RC network often needed to compensate high frequency switchers. The 5430 comes in fixed voltage models, but I went with adjustable this time.

Using TI’s SwiftDesigner to generate a reference design, I drew that up in Eagle. Their reference design specified solid tantalum capacitors, which have properties that lend themselves well to switchmode applications. However, having priced 100+ uF tantalum caps, I’ve decided to use aluminum electrolytic instead. To offset some of the short-comings of aluminum caps, I have connected several in parallel. The main disadvantage is ESR, and wiring caps in parallel cuts the ESR dramatically.

Loosely following TI’s reference layout, I came up with this design. I drew the design using the top layer, and ended up flipping it over when I assembled it. It doesn’t really make a big difference, just as long I remember to solder everything in a mirror image of what’s shown on the screen. For example, the screen shows the input stage on the right, but on the prototype, the input stage is on the left. The reference design called for a single 100uF solid tantalum capacitor rated at 25 volts (I spec’d 14v as VinMax). It also called for a 47uF tant as a bypass cap for the IC. So instead, I went with two 100uF 25v electrolytic caps and one 47uF 16v cap. I realize 16v is cutting it a bit close, but it’s all I could come up with, and this is only a first pass. The output stage called for a single 220uF 10v tant, instead, I drew room for three 100uF caps but only installed two for now. The chip needs a bootstrap cap to help it start-up with low input voltages. The datasheet called for 10nF, so thats what I used. The voltage divider is a 10k resistor coupled with a 10k pot. A resistor and LED were added to show “power on”. The three pin terminal at the bottom is tied to the enable line. Enable should float for normal operation and be pulled low for shutdown. I used a big ‘ol coil I had laying in the parts bin, it was labeled 22uH but the actual inductor is not marked. Looking at the size of the bobbin and heft of the wire, I’d say this inductor can handle some serious current. A three amp schottky diode completes this bit of kit.

With one set of fingers crossed, I hooked the switcher up to a wimpy 200mA 12v wall-wart, used for charging a screwdriver. To my relief the LED came on, nothing started smoking, and the ammeter read 10mA on the 10a scale (later I re-read 13.59mA on the 20mA scale). The datasheet claims 3-4mA of quiescent current, so the switcher is taking 10mA at 12v to supply 20mA at 5v. Although I haven’t done the algebra in the datasheet, comparing watts in to watts out puts the efficiency in the not-bad to pretty good range. (100mw / 120mw = 83%). Better still, when I connected my Dell PDA to the switcher, it was able to supply as much current as the Dell could ask for, without raising above ambient temperature. With a low internal battery, cpu set to 400mhz, external wifi card inserted, backlight full-on, the Dell peaked at 1.1 amps. I left it laying on the bench and watched a movie for a few hours. Coming back, the dell was completely charged, and the output had dropped to about 170mA.

My next goal will be to miniaturize this circuit as best I can, hopefully to fit it into an altoids tin which holds my lithium cells so very nicely.