Tuesday, July 07, 2015

Cheap solar powered Arduino - Part 1

Most of the Arduino projects in mind live outdoors and involve wireless connectivity. This makes power an issue. Solar is an option, but local cells aren't cheap, and cheap cells take forever to get her. I wanted something cheap and readily available.

In this episode, we explore using $0.97 solar yard lights from Wal-Mart. For less than a buck, we get a 1.2V solar cell, a NiCad storage battery, and a few LED's and power sense boards for the junk box.

Power Availability:
Each unit contains a single 2/3 AA battery, with a 150mAh capacity.  We will be running four in series, to power the Arduino with 4.8v.  We don't know the output of our no-name solar cells, but we can guesstimate from their purpose.  As designed, the lights are off  during the day, and illuminate at night.  The LED's generally die overnight, indicating the 150mAh battery is significantly depleted. Since there is no photodetector present, the circuit charges the battery until the solar output falls off, indicating darkness.  The average day in North America is 16hrs, so our solar cell needs to provide >9mA. It doesn't appear our circuit is sophisticated enough to provide overcharge protection, so it probably doesn't provide much more either.  So until we know better, let's assume 10mAh. That's not much...

For simplicity, I just removed the circuit board and wired the solar cell and the battery in parallel, then the 4 units in series.  This gives us a 4.8V weathersafe power source.  If needed we can gang more modules together in parallel, but in the spirit of keeping this cheap, I'd like to avoid that.

Next Issue:
In Part 2, we will see how long our charged assembly will power our Arduino R3 Uno running blink, and a basic webserver running the Ethernet shield.