In the second part, we learned how to charge an electric double layer capacitor through the capacitor mechanism. We learned that it is possible to charge an electric double layer capacitor in a short time. Here, we will use this characteristic to charge it with a solar panel, while supplementing any deficit with power from the electric double layer capacitor to steadily power the Arduino.
Expected time to complete: 60 minutes
This time, we will use a 25F, 5.4V electric double layer capacitor.
Picture 1 A 25F, 5.4V electric double layer capacitor
We previously used a 1F, 5.4V electric double layer capacitor. Therefore, the capacity is increased by approximately 25 times. At full charge, it can power 1 LED for more than 1 hour. Naturally, it will take more time to charge it. Let’s roughly calculate how much time it will take to charge it.
To calculate the charging-time with this equation, we know that the voltage from the solar panel is 5V, and we will assume that the current is 100mA.
t = C × V ÷ I → t = 25F × 5V ÷ 0.1A
t = 1,250 seconds = Approximately 20 minutes
Considering that charging will be completed in 2-4 minutes for solar panels with a current output of 500mA or 1A, we can see how short the charging time will be for the electric double layer capacitor.
Let’s start building to charge the electric double layer capacitor with the solar panels. The large capacitor shown in Figure 1 is the electric double layer capacitor. This circuit puts out a constant 5V voltage from the two serially connected solar panels through a 3-terminal voltage regulator. The electricity generated by the 3-terminal voltage regulator is connected to the electric double layer capacitor and an LED.
Since the electricity generated by the 3-terminal voltage regulator flows toward the path of least resistance (to a lower voltage), it will flow toward the empty electric double layer capacitor. Through this process, the electric double layer capacitor will gradually get charged, thereby increasing the voltage. After a while when the resistance of the electric double layer capacitor becomes greater than for LED, electricity will begin to flow toward the LED from the 3-terminal voltage regulator, and the LED will light up. Therefore, in order to charge fully charge the electric double layer capacitor, the load voltage for the LED circuit should be about 5.3-5.4 V.
Figure 1 Circuit charging the electric double layer capacitor with the solar panels
Picture 2 Circuit charging the electric double layer capacitor with the solar panels
Let’s work on the parts to supply power to the Arduino. Since the recommended input voltage for the Arduino Pro Mini (3.3V) is 3.35V-12V, the 3-terminal voltage regulator (3.3V) used in No. 25 would probably work fine. This time, since a 5V 3-terminal voltage regulator is being used, 3.3V will be supplied from the Arduino’s Vcc pins. However, it is possible to obtain 5V output from the RAW pins, which could be useful for other parts.
In this circuit, Arduino’s RAW pins are connected in the output instead of the LED. As we see before, because the initial load voltage of the electric double layer capacitor is low, it will get charged. When its load voltage exceeds that of the Arduino, the current will start to flow toward the Arduino.
Furthermore, when the power from the solar panels becomes weak, the voltage stored in the electric double layer capacitor becomes greater, such that the current begins to flow from the electric double layer capacitor to Arduino.
Figure 2 Arduino circuit for the solar panels and electric double layer capacitor
Picture 3 Arduino operation circuit for the solar panels and electric double layer capacitor
Since we have managed to establish a stable power source circuit to the Arduino, let’s try manipulating a variety of circuits. First thing: flickering an LED. We will try this by connecting the LED to the pin 9.
Figure 3 Flickering an LED on the power charging circuit
Picture 4 Flicker an LED on the power charging circuit
It definitely lit up! Let’s apply the charging-time equation to examine how long it takes to deplete the power in the capacitor, and use a tester to check how much current was consumed. When the LED is off, the Arduino consumes about 7mA, and when the LED is on, it consumes about 12mA. Therefore, by a rough estimate, it would probably last 2-3 hours. When it is actually turned on, it seems to be more useful than initially imagined.
Next, let’s try powering a servomotor. Motors tends to consume a large amount of power, therefore it may not last long.
Figure 4 Powering a servomotor in the charging circuit
Picture 5 Powering a servomotor in the charging circuit
While the servomotor was in motion, the consumption was about 140mA. This means that it is possible to power it for about 10 minutes, such that a servomotor could be used to control a switch via a stored power.
We powered the Arduino with a circuit that is charging an electric double layer capacitor from the solar panels, and then stored the power when the charge from the solar panels becomes weak. While more work is required to efficiently charge and use the power, its ease of manipulation opens it up for a variety of applications.
The plant-watering machine from “Using Arduino with Parts and Sensors – SD Card Part 2” was modified using the circuit built here. Ideally, this unit should have been placed by the window to expose the plant to the sun. However, dragging an extension cord across the room was less than desirable. By incorporating the circuit, it was possible to independently operate Arduino and the plant.
Picture 6 Plant-watering machine
Aside from the above example, it is possible to operate Arduino without the need for a battery or an outlet, using the solar panels and an electric double layer capacitor, as we did here. Try applying it to a wide variety of things!