I kept talking about it here and there in the forum and the blog. And finally now it’s for real. Parts have been sitting around for weeks and I finally managed to free up some time and get a few assembled, take some shots and put it up on the shop. I know a lot of people asked “how about if you have projects that require 5V because of some special sensors”. So this little board came about for that specific reason. And since I was at work generating 5V from almost nothing, why not add a lipo charger, a voltage level detector, a little prototyping area and a switch (but maybe not the kitchen sink) ?
The charger chip is the popular MCP73871, charges at 500mA via USM mini-B and has a charging status LED. The booster is the mighty TPS61220 adjustable output boost regulator. The output voltage is 5V by default. It can be switched to 3.3V if wanted, by means of a solder jumper:
Even though it works, you should avoid trying to get 3.3V output from a 3.7V lithium battery, because … well … it defeats the purpose of it. Try getting 3.3V output from something much less than that, say a AA/AAA cell, that will put this board to good use.
I’ve started using this to power MotionMote from a small 400mA battery since the PIR sensor requires at least 5V, and Moteino will work great with 5V input (on “VIN”). So that’s a great application, and battery voltage monitoring comes included, sweet. The little lipo won’t last a long time (actually I don’t know how long … we’ll see), but when it does, I can see it from the incoming battery level indicator, and then just quickly recharge it. Not awfully inconvenient. The battery voltage monitor is just a VCC-1Meg-470K+0.1uF-GND circuit which divides the voltage and feeds it into Moteino analog pin A7 where you can read it as analog and interpret it according to what battery you got hooked up. A formula that I use for batteries up to 9V is:
int batteryReading = analogRead(A7); float battV = ((float)batteryReading * 3.3 * 9)/(1023*2.976);
As you can see there’s also a small prototyping area. The Moteino can be mounted above or below, your choice, just keep the power pins aligned of course. The switch should be OFF to avoid loads when charging and let the charger exert its love on the battery uninterrupted.
At this point it hasn’t been extensively tested but I’ve had no issues using it in my projects. And hey this little board will resurrect your AA batteries from among the many dead you surely have around (you didn’t throw all those away right?). Getting 5V/3.3V from as low as 0.7 is not too hard for the Moteino Power Shield.
Eagle schematic and layout are available on Github.