I’ve been working with some high power LEDs around here. I also ride my bike to work. The cheap bike lights I’ve owned kind of suck. In fact, some barely illuminate the road. And none of them flash groovy colors. Last week I bought a $20 Schwinn light (pictured above). I really like the shape of the light, and although it is an improvement over my last lamp, it’s still pretty tame in the light production department. So I’ve decided to pump up the lumens, taste the Technicolor rainbow, and build a disco bike light. And if I use the Schwinn body I can avoid some of the mechanical design effort.
The Schwinn bike light breaks down into several pieces. The main body is the black tube where I’ll need to fit my electronics. Normally it carries three AAA batteries (shown on the left). I’ve added a AA battery for size comparison, because that ends up mattering later on. There’s also a screw-in base that has the on/off/flasher circuit in it. It’s the part with the spring attached that presses against the negative terminal of the battery pack. This part is a little strange. It operates without the battery pack and provides an open, short, or 500ohm resistance from the case to spring (ground). I’m guessing there’s a microcontroller embedded in there, but I swore not to take it apart (I’ll need that light in the coming weeks). The last piece of the light is the aluminum LED holder. It’s a thin threaded inset that connects the single white LED to the positive battery terminal.
I want to use the same parts we have on our BM014 Super Bright RGB LED Module, because, well we have them in stock. I also want to be able to program color and brightness settings, so that means a microcontroller and user interface. I’ll also have to replace the 3-AAA batteries with something else. 1-AA Lithium Ion battery would provide a similar voltage, and give me some extra room for circuitry.
The first step in this design is realizing that I’ll need to fit the Lithium Ion battery, a circuit board, and USB connector into the main body of the light and onto a PCB (for charging and communicating with the microcontroller on board).
I turned to Sketchup 8 to do some simple modeling. If I mount the USB connector directly under the rear battery mount everything works well. To do this I would have to clip and sand the through-hole tabs on the battery mount (PN: BK-92-ND at Digi-Key). If there’s no room for it anywhere else I can go that route. The image below shows what I’m going on about.
The circuit board that fits inside the tube is pretty big, I calculate it at 0.725”x2.0”. So I think I’ll have room to move the USB connector. On the top side of the circuit board will be the battery mounts. On the other will be the USB-to-serial converter, a Lithium Ion charging circuit, and an LED drive circuit. I can rotate the USB connector and get it to fit further in from the battery clips. I would also have to notch the circuit board so a USB cable will fit into the USB connector. Maybe I can find some surface mount battery tabs?
Lastly, since the housing of the bike light is conductive, I’ll have to make sure the battery terminals can’t touch the sides of the enclosure. That’s tricky, but I have some ideas about how to do it with the PCB. I’ll discuss that in the next blog on this topic.