Since the location of the Garduino has no access to power, I decided to Solar Power it.
I bought this 12 V 10 Watt Solar Panel off eBay for about $50. Since mounting brackets for these panels cost nearly as much as the panel, I decided to make my own with a piece of Home Depot Aluminum, a coupe of U Bolts, and some small L Brackets.
Normally, from what I’ve read, you want to angle the panel at the same angle as your latitude, but it was a whole lot easier for my band saw to cut the aluminum at a 45 degree angle, rather than the 39 degree angle my latitude would indicate. I’ll have to live with that inefficiency.
Since I want the device to run even when the sun is down, I also needed a battery. I just picked up a cheap 12 V 7Ah battery off the internet somewhere for that job. To tie the Arduino, battery and panel together, I bought a solar charge controller off Amazon. The Kintrex SPC0601 was the cheapest, lowest power one I could find, but it fits the power levels that I am working with. It basically has 3 connections, one for the battery, one for the panel and one for the load, and it’s job it to manage charging the battery during the day, and running the load off the battery or the panel as appropriate.
This picture shows the Kintrek and the Battery in the water proof box. It also shows the wires to the various sensors as well as the sensor controller board, which I will discuss in a later post.
One thing to note is the 2.1mm power jack in the picture. I originally plugged the Arduino Uno directly into that jack, but the voltage regulator on the Arduino got very warm. It got warm because the voltage regulator is very wasteful of my limited power. Assume that my Arduino and sensor circuitry draws 100mA (probably a little high, but it makes the math easy). Since the Arduino runs off of 5 V, the voltage regulator has to drop 7 volts (12 – 5 volts) at 100mA, and it has to throw off 7 V X 100mA = .7 Watts of power as heat to supply the Arduino with 5V X 100mA = .5 Watts of power. That means my efficiency is .5 W/1.2 W (.5 W+ .7 W) or about 42% power efficiency. That isn’t going to cut it, considering that this setup will also end up powering an XBee that has a max draw of around 200mA.
I started looking into building a DC switching power supply of some sort when I ran across these Universal 12 Volt USB Car Chargers for about $2 on Amazon. Tearing one open, I saw a nifty little switching power supply with the added bonus of a USB connector so that I could simply use a USB cable to the Arduino for power.
For $2 you don’t get a detailed datasheet that describes things like the maximum current capacity, but the D34063 chip at the center of the supply is rated at 1.5 A, so I feel pretty comfortable that this should be able to handle the load. Since a switching power supply is turning on and off the current at a pretty high frequency, it is difficult to determine an effective current input to the device, so it is hard to calculate the efficiency of the power supply. When I’m bored, I might try to hook up my oscilloscope to it to try to get an idea of the efficiency of the device, but I’m sure it is way better than 42%, because the device does not get even a little warm in use.
The final thing I’d like to discuss is the water proof container. I initially used a Tupperware, some glue and electrical tape, but it was a complete failure. A couple of rainy days later, I had water pooling in the container, and a ruined Solar Charge Converter. Luckily the Arduino stayed dry, but I had to find a better container for the Garduino.
This is the $20 Dri-Box Waterproof Connection Enclosure, and it has been working pretty well so far. If you notice the earlier picture, where you can see the inside of the box, you can see the channels that the cables run through are filled with a soft rubber which seal around the cables. I have yet to see any evidence of water inside the container, and we have had some deluges in the time it has been out there.