Arduino Irrigation System #4

[Part 1] [Part 2] [Part 3] [Part 4] [Part 5 – ALISS] [Download]

The Solar System

Billions and billions of stars. . . oh, sorry, I was having a Brian Cox moment.

As I mentioned in Part 1 the nursery is not connected to the Grid, so to power the irrigation system, we need to get our Hippie on.

Getting ‘our Hippie on’ is much easier said than done. You can’t just plonk down a Solar panel and a battery and expect it to work. The very first thing you must do is ‘Size your System’. This involves spending lots of time working out how much power your system is going to use, when it’s going to use it and how much you can afford. Doing this tells you how big your solar panel(s) need to be and how big your battery bank has to be.

Once you’ve worked out how much energy your system uses, the next step is to re-evaluate the system and make it more energy efficient; this is where the 3-way solenoid valves come in. Using 3-way valves means that only 1 valve has to be on at any one time, and this reduces maximum simultaneous load on the batteries.

Sizing A Solar System

I searched the internet for a very long time looking for a detailed explanation on sizing a solar system. What I wanted was the formula, but most sites only provided a calculator. Eventually (and I really do mean eventually) I came across this how-to article on I’m going to quote the formula from that article in a moment, but even though I have done that, there is still a whole load of useful information and interesting products on their website, so do go and have a look.

The maths goes like this:

  1. Device Amps X 12 = Watts (if applicable)
  2. Watts X Hours used per day = Watt hour per day (Wh/day) (1000Wh == 1kWh, what your electricity company calls a Unit of Electricity.)
  3. Wh/day ÷ Hours of useable sunlight = Panel size (round up)
  4. Wh/day X 7 = Weekly wattage requirement
  5. Weekly wattage requirement ÷ 12 = Amp hours
  6. Amp hours X 2 = Minimum battery size
  7. Panel size ÷ 16.5 = Amps produced (used for charge controller)


To size the system for the nursery we do the following:

  1. 0.3A X 12 = 3.6W (Valve draw)
  2. 3.6W X 2 = 7.2Wh/day
  3. 7.2Wh/day ÷ 4 = 1.8W (rounded to 2)
  4. 7.2Wh/day X 7 = 50.4W
  5. 50.4W ÷ 12 = 4.2Ah
  6. 4.2Ah X 2 = 8.4Ah
  7. 2W ÷ 16.5 = 0.12A

Or, as I put it in my Field Notes Tradesman Edition:

Size A Solar Array

So what does this tell us?

Equation 3 tells us we need a 2W Solar Panel.

Equation 6 tells us we need at least an 8.4Ah Battery.

Equation 7 tells us we don’t need a particularly heavy charge controller.


Now that we have this information, we need to learn about Solar Panels.

Series / Parallel and Series-Parallel

Again I spent a long time researching solar panels, how they work, how to connect them together and their advantages and disadvantages.

For the most part, it isn’t actually necessary to know how they work, although it is very interesting. What is important is knowing how to connect lots of smaller (and cheaper) panels together.

Just like batteries, there are three ways to connect Solar Panels together.

For this exercise, assume that you have two Solar Panels, and you want to connect them both to one battery. Assume also that the Panels are each rated at 3V and 2A. Each panel has a Negative (black) side and a Positive (red) side.


To connect the Panels in Series you connect the Black side of Panel 1 to the Negative terminal of the Battery.

Next you connect the Red side of Panel 1 to the Black side of Panel 2.

Finally, you connect the Red side of Panel 2 to the Positive Terminal of the Battery.

In a Series setup like this, you add the Voltages together, meaning that this setup provides 6V at 2A.


To connect the Panels in Parallel you connect Panel 1’s Black to Panel 2’s Black.

Then Panel 1’s Red to Panel 2’s Red.

Then connect the Battery’s Negative to the Black side of the circuit, and the Positive to the Red side.

In a Parallel setup, you add the Amps together, meaning that this setup provides 3V at 4A.


Finally, in Series-Parallel, you connect the panels up using both methods.

For this step, we’ll need 4 Panels instead of just 2.

Step 1 – Take Panel 1 and connect it to Panel 2 in Series.

Step 2 – Take Panel 3 and connect it to Panel 4 in Series.

Step 3 – Connect the Panel 1’s Black to Panel 3’s Black.

Step 4 – Connect Panel 2’s Red to Panel 4’s Red.

Finally, connect the Battery’s Negative to Black, and Positive to Red.

This setup gives us 6V and 4A. In other words, the best of both worlds.


Of course, there are losses involved in all these setups. Pushing energy into a battery causes losses (as does taking it out again), long, or poorly connected wires cause losses, heat and dirt cause losses, the list goes on. Just take that into account.

(If you want to learn more about losses in a Solar System, watch video #3 from this YouTube Playlist by MJLorton. And then subscribe because he clearly knows what he’s talking about and I have found his videos particularly helpful.)


Build A Solar Array

1x 16.5V 3W Solar Array and 1x 6V Solar Panel

1x 16.5V 3W Solar Array and 1x 6V Solar Panel

The image above is the finished and installed Solar Array I built using nine 5.5V 1W Panels which I bought from CoolComponents. They are connected in Series-Parallel to produce (at peak time) 16.5V @ 3W. Remember that our calculations earlier told us we needed at least a 2W Panel. (There is also a smaller, single 6V Solar Panel. That charges the 3.7V LiPo Battery that powers the Arduino (6V Panel from Proto-PIC).

In the image below you can see the underside of the Array. (The terminals are protected with Sugru.)

The Red wires at the bottom connect the bottom row of Panels Positive terminals together.

The Blue wires connect the bottom Panels’ Negative terminals to the middle Panels’ Positive terminals. And the Middle Panels’ Negative terminals to the top Panels’ Positive terminals.

The Black wires connect the top Panels’ Negative terminals together.

Finally, in the top left you can see a long Black wire, and in the bottom right you can just see a Blue wire leaving the frame. These are the Array’s Negative and Positive wires. (Blue because I had run out of Red.)

Arduino Solar Panel Array

The white stuff is Copydex which I used to adhere the cork to the bottom of the Array, to lift the whole unit so that the Array sits flat on the mounting board, rather than rest at various angles due to varying sizes of Sugru blobs. In the end, it looked like this:

Arduino Solar Panel Array

Copydex was then applied to the other side of the cork pieces and then the whole unit was flipped over and stuck to the grey mounting board (Which was actually a Mirror.)

Now it is simply a case of connecting the Array to the Charge Controller, or directly to the Battery if you’re that way inclined.


In Part 5 we’ll finish up this version of the Irrigation System and we’ll see it in action.


FYI: I have already prototyped version 2 which uses an Arduino Mega, a 4×4 Keypad and an LCD Display. And I plan to include a Servo for turing the Tap on/off and an Adafruit Fona for remote operation from anywhere in the world!


[Part 1] [Part 2] [Part 3] [Part 4] [Part 5 – ALISS] [Download]

Author: Dan

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