An IoT project: Monitoring soil moisture (Phase 3) – Adding Watering Capability (Part 1)

In the previous articles, we covered the development of a soil moisture sensing system and the addition of a WiFi module so that the soil moisture information could be sent out. Phase 1 of the project is covered here while Phase 2 (Part 1) is covered here. If you are wondering why there’s no part 2 of Phase 2, it’s taking longer than I anticipated to implement whereas part 1 of Phase 3 is much easier to implement.

And as mentioned in our first article, we will be adding plant watering capabilities in Phase 3 of the project. We will call this the water pumping station.

To do this phase of the project, we will be needing a few additional pieces of hardware to build this station.

Below is the list:

  1. 2x 12v peristaltic pump (also known as Dosing pump)
  2. Grove I2C Motor Driver
  3. Wires – Solid core 22AWG and Stranded (11 to 16 strands)
  4. 12v DC power supply
  5. Stripboard
  6. Soldering kit
  7. 12v barrel connector plugs (5.5mm x 2.1 mm female and male version)

Now that we have the hardware established, let’s get started.

Setting up the power module

This water pumping station will be connected to the existing soil moisture monitoring system. Given that the Arduino Uno accepts a 12v supply through its barrel connector, we could use the same power source as the water pumping station.

To achieve that, we could use a combination of stripboard, wires and barrel connector plugs. Sadly, there were no pictures of how I develop the power module other than the final picture when it’s already installed in the housing. I had to take apart the housing just to take a picture of it.

As you can see, two different type of barrel connector are hooked up and a pair of wires that went to the motor driver board. The female barrel connector is meant to receive the 12v power from the power supply while the male barrel connector plug is meant to connect to the Arduino.

On the other side of the stripboard, the wires are connected in series with no other fancy electrical components. In hindsight, it might have been better if I added a few components such as switches and LED but that would be for some future project.

Implementing the motor driver and adding motors/pumps

Arduino Uno alone can be used to turn on a motor with a combination of:

  1. Digital and analog pins
  2. Electrical components such as resistor, diodes and transistor
  3. External power supply
  4. Breadboard or stripboard

You can even find a tutorial for the above here. What this does is to use the Arduino to generate a signal to control the transistor to run the motor at full speed or turn it off. It does not allow for reversing the motor spin or controlling its speed.

Do not power and drive motors directly from any of the Arduino pins. They do not provide sufficient power to drive motors nor do they have the necessary protection against counter EMF from the motor.

Alternatively, we could go with a motor driver board that comes with all the necessary components installed that will allow us to control multiple motors with ease. It could also allow us to control the speed of the motors and the direction of the speed.

And the second option is what we will go with.

The Grove I2C Motor Driver v1.3 by Seeed is a good choice as it supports driving two motors from its twin channels. The motor driver comes with the L298 IC chip which is a dual full-bridge that support high voltage and current designed to drive inductive load such as relays, solenoids, DC and stepping motors. It also has an ATmega8L chip to enable I2C connection, making it very compatible with Arduino.

We can connect the motor driver to an Arduino using the I2C connection via the Grove port and some jumper wires. But since I don’t have a spare Grove shield, I have to go with the alternative connection. The SCL and SDA pins on the Arduino Uno are A5 and A4 respectively. With that in mind, we can connect the yellow and white wires to those pins. Then connect the 5v and ground pin.

Now we are ready to test the driver with an actual motor or pump.

But first, we will need to connect the wires to the terminals on the motor.

We could also solder the wires to the terminals to ensure a more secure and stable connection. You can see the result below after soldering and the pump is installed in the housing.

After connecting the pump to the motor driver, we could start testing whether the driver works and understand better how to use the hardware.

First, connect the Arduino to the computer. Then upload a BareMinimum sketch to the Arduino before we connect the motor driver. This will remove any existing programs on the Arduino and ensure there is nothing interfering with the driver.

Once the upload is completed, remove the black jumper on the driver. This step is necessary before we power up the driver with the 12v supply as failure to do so will mean 12v going into the Arduino via the 5v pin. Since there’s no electrical protection through that pin, we could fry the Arduino. Once the jumper is removed, we can connect the motor driver to the 12v supply. Alternatively, you could always use a battery to barrel jack adapter such as this and hook up a battery with voltage anywhere between 9v to 12v.

The image below shows the motor driver hooked up to the Arduino with the black jumper removed. The pump is also connected but it’s still in its bubble wrap.

Experimenting with and testing the pump

Based on the specification of the pump, it could do 100ml/min. However, it’s always better to test it and see for yourself the performance of the pump. But before we could do any experimentation, we will attempt to get the motor driver running.

The green LEDs should light up once the driver is powered and running.

But, if you see red LED light up alongside the green, it means that the driver is not initialised properly.

To fix it, simply press the white RESET button on the driver itself while it is connected to a powered Arduino. Everything should work after.

During the various testing session, I noticed that the motor driver always have some initialisation problem after I deploy a Sketch to the Arduino or when it is freshly powered up. That meant I will need to press the RESET button at least once if I want the driver to work properly. This is a problem but we will cover more later as it also affect the housing design. For now, we will continue with figuring out the water flow rate.

To do that, we will need a container filled with water, a measuring beaker and a small medicine cup. After connecting the silicon tubes to the pump, insert the suction tube into the water container.

But how do we know which is the suction end?

For the Gikfun pump that I’m using, the suction end is on the right side if the two tube connectors are facing you. Alternatively, you can determine the suction end by running the pump straight from a 12v supply and check which tube takes in the water. Always ensure the other tube is placed in the measuring beaker. We don’t want to spill water all over the place, especially not when electronics and electricity are involved.

Once we know which end of the pump takes in water and output water, we can start testing how much water is being output.

We can upload a simple Sketch that spins the motor in the pump for a certain amount of time.

#include "Grove_I2C_Motor_Driver.h"

#define I2C_ADDRESS 0x0f

void setup()
{
  Serial.begin(9600);
  Serial.println("Starting motor");

  Motor.begin(I2C_ADDRESS);
  Serial.println("Motor Initialised.");
  delay(2000);
}

void loop() {
 
  Serial.println("Running pump 1");
  Motor.speed(MOTOR1, 100);
  delay(15000); //15 seconds
  Motor.stop(MOTOR1);
  Serial.println("Motor stopped.");
  
  delay(1000UL * 60UL * 5UL);
}

For the test, I went with 15 seconds and ran the pump at 100% for my first experiment. The reason behind for the 15 seconds was based on a gut feel that anything lesser would not be sufficient to water the plant and was kind of pointless to check them out. The 5 minute delay is there to enable us to take measurement of how much water is dumped into the beaker.

Since the smallest value on the beaker was 100ml, there wasn’t sufficient water in the beaker to measure. This is where the medicine cup comes in since the smallest value on it is 1ml and the largest value is 15ml.

I poured the water from the beaker into the medicine cup until it reached the 15 ml mark and checked the beaker. There was no water left in the beaker.

The delay is then increased to 25 seconds and I repeat the measuring process. It turns out the pump put out about 25ml in 25 seconds. This is completely different from what is specified.

If the pump could do 100ml/min, that means it could do 1.6ml per second. At 25 seconds, I should see 41ml of water in 25 seconds and not 25 ml. This just prove that taking actual measurement is always necessary.

With this discovery, I could then determine how long to run the pump for based on the size of the pot, the type of soil and how dry is the soil. This shall be the topic for another article.

Housing the pump station and getting ready for integration

For continuity in design, I will be using Lego bricks again. Previously, I got the Lego Brick Bricks Plate version which comes with 1,500 pieces and some base plates with the intention of doing this project. And I also had left over Lego bricks from my other IoT project that I will be using first.

After choosing the right base plate, the power module was installed first. The bricks were installed with the idea that they should hide the power module since it’s relatively ugly. Once that’s done, the motor driver goes in and the bricks are added.

Below is the initial design that I had before I realised it couldn’t support two pumps.

After some more hours of work, the first pump is now installed and walls went up. The motor driver was moved to the center of the base plate.

This is the side view. As you can see, cable ties are used since the pump has a dimension that’s incompatible with the Lego bricks.

After adding the second pump, the pumping station is now mostly completed.

It was only after the housing is done when I came to the realisation that I need a quick way to access the RESET button as I will be integrating it with the soil moisture monitoring system and will need to deploy new Sketches every now and then. And, I also need to take into account whenever there is a lightning storm, the circuit breaker in my house will trip. In that scenario, the motor driver will probably need a reset.

The good news is there are ISP headers on the board that we can use to perform the reset.

Now, the RST is pulled high by default. To perform the reset, we need to connect it to ground, thereby pulling it low. And we can achieve that with a push button, connecting the RST pin to ground.

So, it was time to take out the motor driver from the housing and solder on the pin headers.

Then, we can prepare a push button and solder the wires on.

Once everything is hooked up, we shall have a quick access reset button.

Then, we will power up the motor driver and test the reset button to make sure it works. As usual, the red LEDs on the driver will light up once powered on. It is because of the driver’s failure to establish the I2C connection. This is when I connected a powered Arduino to the driver and then pressed the reset button. After a while, the red LEDs disappear and that means the I2C connection is established.

It was discovered later that there is a bug with the motor driver’s firmware. It crashes when something that is not hardcoded in the firmware is sent to the driver. This causes the I2C connection to fail. The good news is that a new firmware has since been made available and we could update it by following the instructions here.

Now that I’ve proven the push button to reset works, the housing is then modified to incorporate the button. I had to use several layers of paper to get the button to sit tightly in the window brick.

With that, we can start integrating with the soil moisture monitoring system. The two lego housing are first combined but we won’t be connecting the motor driver or powering it up as we still need to update the sketch program currently running in production.

Here are some picture of the upgraded system.

This marks a good point to stop this article. In the article covering Part 2 Phase 3, we will look at how we setup the tubes around the plant and update the existing sketch program. And let’s not forget that the article for Part 2 Phase 2 is also in the works. In that article, we will look at how to upload telemetry data from the Arduino to a dashboard.

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