At the OLPC summit in late October 2013, I gave a demonstration of an Arduino based current monitor. At the time, I was more excited about the schoolserver on the trimslice which was streaming Internet In A Box to a portable LED battery powered projector.
But in retrospect, I’m thinking that the battery monitor, which I also was demonstrating at the Summit, is more needed in the developing world.
During the December holidays, I started thinking about the XOduino that Scott Ananian designed which had a total parts cost of $10 or so (surface mount parts are cheap). I dreamed that if the battery monitor circuit could be added to his design, for a total parts cost less than $20, the product could be sold for $60-90, at an attractive price point. It was a learning experience to start from Scott’s circuit board design, using Cadsoft’s Eagle circuit board design tool, stripping off all the unwanted parts, an adding those needed for current monitoring.
It turns out, for me, that soldring the CPU (44 pins) was infinitely easier than resistor networks with only 8 pins. I concluded that hand soldering was not really an option (I was thinking cottage industry in Haiti).
So I reset my thinking. At about the same time I noticed that Amazon had Arduino Leonardo knock offs available for $13 (see knockoff Leonardo). Of course I ordered 2. And I discovered that they would be shipped from Hong Kong, and would arrive in a month. But if these boards turn out to function adequately, the standard Arduino, with a shield, composed with through hole parts, seems like a better solution.
On Feb 13, I submitted a through hole design to the printed circuit fabrication shop I like (oshpark.com). This company gives about 15 day turn around on 3 boards for $30. The plated through holes make it so much easier to put together a functioning prototype. Otherwise I have to solder on both sides of the board at the critical pads where current must transfer from one side to the other (really hard underneath connectors).
The schematic for this Arduino Shield: current_monitor_v2. The design documentation from which I drew inspiration can be found in this application note.
Many battery experts believe that the life of lead acid batteries can be greatly extended by using very short pulses of high current. This circuit, called a “desulfator” is inspired by this reference at http://fucimin.altervista.org/desulf/desulfator.pdf. I am just about to have a circuit board built for this circuit: battery-nurse as a companion for the Arduino shield mentioned in the previous paragraph.