When I’m building Solar Shrubs and other solar-powered creations, I often scavenge cells from various off-the-shelf devices such as solar garden or security lights. But these cells are rarely labeled as to their voltage, current, and power output.

So it’s off to my bread-boarding station. First, I put each cell under a lamp and connect it to my multimeter to get the volts. Then I build an I-R curve by measuring the voltage across a series of different sized resistors. This takes a lot of time and effort and I still don’t know what the values are in sunlight. I could drag the multimeter, breadboard, and all associated components outside, but that would be a hassle and it’s still very time consuming if I have a lot of cells to test.

My solution was to build an Arduino-based, handheld Solar Cell Tester. This tester can read up to 15 Volts at 1 Amp. Now I can carry one device outside, attach a single solar cell or a group of cells in series or parallel, and read voltage, current, and power quickly and accurately!

Here’s how you can build one too!

NOTE: At first glance this device looks very similar to Ladyada’s impressive Portable Solar Charging Tracker, But mine is actually more limited; only testing the cells, not lithium batteries and charging circuits. I used Adafruit’s enclosure and protoshield for this project, but the calculations, circuits, and program are all mine (except for the excellent help, as always, from the Arduino on-line community!)

Step 1: Materials & Tools Needed

Materials Used:

(1) Arduino Uno
(1) Arduino Proto Shield (Adafruit #51)
(1) White Enclosure for Arduino (Adafruit #271)
(1) Parallax Serial LCD Panel (Parallax #27977)
(2) 10 ohm 10W Power Resistors (for current sensing circuit) (Radio Shack #271-132)
(1) 2K ohm resistor (for voltage sensing circuit)
(1) 1K ohm resistor (for voltage sensing circuit)
(1) DPDT Sub-Mini Toggle Switch (Radio Shack #275-614)
(1) SPDT Slide Switch (Radio Shack #275-409)
(1) 1/8″ Phone Jack (Radio Shack #274-251)
(1) 1/8″ Phone Plug (Radio Shack #274-284)
(2) 14″ Insulated Test Leads (Radio Shack #278-1156)
(2) AAA Battery Holders (2 cell holder) (Radio Shack #270-413)

Tools Needed:

Soldering Iron
Solder
Helping Hands
Wire strippers
Side cutters
Dremel

Step 2: The Circuits

A DPDT toggle switch (SW1) serves as the on-off control which disconnects the 4-AAA batteries from the Arduino as well as the “Cell Under Test” from the sensing circuits. The cell under test is connected via red and black test leads attached to a phone plug and plugged into a 1/8″ phone jack on top of the tester.

The tester includes two sensing circuits; one for measuring voltage and one for measuring current. A SPDT toggle switch (SW2) connects the cell under test to these circuits individually.

The Voltage Circuit: The Arduino analogRead command reads voltage up to +5V and returns an integer between 0 and 1023. In order for the tester to read up to 15V, you’ll create a voltage divider that consists of a 2K (R3) and 1K (R4) resistor. The voltage across R4 is one-third the source voltage so it can read 0-15V. (NOTE: You can use any two resistors with a 2 to 1 ratio).

The Current Circuit: Since the analogRead command returns a value of 0-1023 (for a max of 5V), each unit is 5 divided by 1024 or 4.9mV. And from Ohms law, we know that the voltage drop across a resistor is equal to the current times the resistance. So the voltage drop across a 4.9 ohm resistor with 1mA of current is 4.9mV. This means we can effectively read the current across a 4.9 ohm resistor using the actual value returned by the analogRead command. There are at least two problems with this strategy.

(1) Because of it’s low resistance, this circuit can produce a high current and consequently too much power for standard 1/4 or 1/2 Watt resistors. They would become dangerously hot very fast. So I recommend using Power resistors with at least a 5W rating.

(2) I looked high and low and couldn’t find a 4.9 ohm power resistor.

The Solution?  Connect two 10 ohm resistors in parallel, which will give you approximately 5 ohms (close enough for this project). The power resistors I used are rated at 10W so they’ll stay nice and cool in this circuit.

The voltage, current, and power readings will be displayed on a 2×16 character serial LCD panel. You can use any LCD panel, but the wiring and program may need to be modified to accommodate it.

For more detail: Arduino Solar Cell Tester