Here’s an update, long overdue. This project is still in work, but I have been diverted to other things lately, and haven’t given it as much attention as I’d’ve liked.
I have received a batch of bigger, better solar cells from BG Micro that I’m working to make into a panel. They purport to provide 4.5V / 90mA, and I have 10 of them, so we’ll see if 900mA is enough to drive the charger and MintyBoost the way I hope. Once I see how they perform, I may try and get more.
Having looked into a number of ways to do NiMH charging, I found a few special-purpose ICs made by Linear that look promising, in particular, the LTC4010 / LTC4011. They can do both the voltage-drop and temperature-rise detection of a completed charge, and they also have some built-in timing mechanisms to prevent accidental overcharging. Once they have arrived and I have made some breakout boards for them, I’ll try them out in the lab.
Introducing MintyStick v3.0
Finally, I have succeeding in producing my own variation on the MintyBoost. I found inspiration on this post (http://forums.adafruit.com/viewtopic.php?f=15&t=18225) on the Adafruit forums. It looks like a stick of gum, and I like it, because it’s single-sided, and because it looked like it would be pretty easy to make at home with my toner transfer setup. I call it the MintyStick, because sometimes I don’t like to spend a lot of time trying to come up with good names for things. It’s version 3.0 to indicate compatibility with the MintyBoost 3.0 (since it ought to support the same devices).
[Edit: I had originally called it the “MintStick”, but people have been referring to it as “MintyStick”, which I like better, so I have updated it.]
I started with the MintyBoost design files posted on Lady Ada’s website: http://www.ladyada.net/make/mintyboost/download.html, where they are available under the Creative Commons Attribution 2.5 license. I had to update the schematic to match MintyBoost version 3.0, and my board layout is rather a departure from the original forum post. First, I had to add the extra resistors in the 3.0 version. Second, I rearranged the components around the LT1302 according to some of the recommendations in the datasheet. I put the power terminal (for the battery) at one end and easily accessible (so I could use a plug, or a 2.54mm terminal block, or what-have-you), and I had to rearrange pretty much all of the other components to make it compact and nice-looking:
Since my goal was easy home production, I made all the traces extra wide, and increased the size of the pads where I could. Here are some images of the first one:
Here’s the PCB traces (all routed on the bottom). I went over some of the traces that didn’t transfer perfectly with a sharpie, so that explains why some of them may appear a bit uneven around the edges (and the big mess around the USB socket mounting holes).
Here’s the top, with my feeble attempt of toner transfer for the label layers. I think I’ll skip messing with this in the future, it did not turn out to be as helpful as I’d hoped. It was hard to line up, and it didn’t transfer cleanly at all.
Here’s the assembled version, from the top. I didn’t have a 3.3K resistor handy, so I made one out of a 3K and a 300 ohm in series. You can see the original footprints for the resistors were too narrow. I updated them on the board to 7.5mm versions, which should fit better.
Here’s the board, populated, from the bottom.
As you can see, it fits nicely in a full-size Altoids tin, with plenty of room for 4 AA cells, or 2 cells and another circuit board (with the charge controller). The next step is figuring out how to charge the NiMH with the solar panel, and to see if I can run the boost converter from the panel directly.
I’ve tried it on several devices, and it works, and is recognized as an approved charger on