Build a Circuit to Automatically Water Your Plants

This project follows the Engineering Design Process. Confirm with your teacher if this is acceptable for your project, and review the steps before you begin.

Calibrate Your Sensor

Before you build a full automatic watering system, you need to calibrate your soil moisture sensor for the soil you plan to work with.

1. Build the circuit shown in Figure 3.
   1. For the soil moisture sensor:
      1. VCC to Arduino pin 8
      2. GND to the ground bus
      3. SIG to Arduino pin A0
   2. For the LEDs:
      1. Red LED and 220 Ω resistor to Arduino pin 2
      2. Yellow LED and 220 Ω resistor to Arduino pin 3
      3. Blue LED and 220 Ω resistor to Arduino pin 4
   
   

   Image Credit: Ben Finio, Science Buddies / Science Buddies

   Figure 3. Breadboard diagram for the Arduino soil moisture sensor circuit.
2. Download soil_moisture_sensor_calibration.ino. Read through the commented code so you understand how it works. A Tinkercad Circuits simulation with the code is also available.
3. Upload the code to your Arduino and open the serial monitor (Tools→Serial Monitor). The serial monitor should read 0 and the red LED should be on when the probes are not touching anything. If you touch both probes at the same time with one finger, the reading in the serial monitor should change, since your skin is slightly conductive.
4. Prepare a bowl of dry soil. You can leave the soil out in the sun or speed up the process by drying it in an oven or microwave. Try to make the soil as dry as possible.
5. Prepare a data table like Table 1.

Table 1. Table for recording sensor calibration data.

6. Push the sensor firmly into the soil, so the probes are almost completely submerged (Figure 4).
7. Watch the serial monitor and immediately write down the sensor's initial reading.
8. The sensor reading may fluctuate at first. Wait five minutes for the reading to stabilize, then write down the final reading. If the reading is still changing, wait another five minutes, then check again.
   Image Credit: Ben Finio, Science Buddies / Science Buddies

   Figure 4. Close-up picture of the soil moisture sensor in the soil.
9. Pull the sensor out of the soil. Dry it off with a clean towel.
10. Put the sensor in a different location in the soil. The measurement will depend on the contact area between the sensor probes and the soil, so make sure you test it at the same depth each time.
11. Repeat steps 7–10 as needed for at least two more trials with dry soil.
12. Slowly add water to the soil until it is completely saturated (cannot absorb any more water).
13. Repeat steps 6–11 for at least three trials with wet soil.
14. Calculate average initial and final readings for both the wet and dry soil.
15. In the code, adjust the values for the wet and dry variables. You will need to pick appropriate values based on your calibration, your soil, and your plant. For example, you might not want to wait until the soil is completely dry to start watering, and you might not want to apply water until the soil is saturated. You may wish to set values in the code that are somewhere between the calibration values you recorded.
16. Finally, adjust the delayTime variable. This variable defaults to 1 second (1,000 milliseconds) to make the calibration process quick, but you probably do not need to check the moisture level of your plant's soil every second! You should think about what value makes the most sense for your plant, but be careful not to make the delay too long. For example, if you only check the sensor once per hour, then when the soil gets dry, the pump will run continuously until the Arduino checks the sensor value again an hour later. This could result in overwatering or even water spilling out of the pot!

Set Up Your Automatic Watering System

Now you are ready to connect a pump and start automatically watering your plant.

1. Connect the pump to your circuit as shown in Figure 5. Note: This image uses a DC motor to represent the pump, because Tinkercad does not have a separate "pump" part, but the wiring is the same.
   1. Put the MOSFET in the breadboard with its pins in three different rows.
      1. Connect the MOSFET's gate (the pin on the left when the writing is facing you) to Arduino pin 11.
      2. Connect the MOSFET's drain (the middle pin) to the pump's negative wire.
      3. Connect the MOSFET's source (the pin on the right when the writing is facing you) to ground.
   2. Connect the pump's positive wire to 5 V.
   
   

   Image Credit: Ben Finio, Science Buddies / Science Buddies

   Figure 5. Pump (represented by a DC motor in this diagram) connected to the circuit. The pump is controlled by a MOSFET since the Arduino's pins cannot supply enough current to drive the pump directly.
2. Download soil_moisture_sensor_pump.ino and upload it to your Arduino. A Tinkercad Circuits simulation is also available.
3. Now you need to physically set up the system for your plant. Your exact setup will depend on your situation, but here are some things to consider. (Figure 2 shows an example.)
   1. How will you power the Arduino? You might not want to leave it plugged in to your computer's USB port for weeks at a time. You can power the Arduino using a wall charger or a USB wall adapter and the USB cable (see materials list).
   2. How will you protect the Arduino and circuit from dirt and moisture? Will you put them in a container? Where will you put the container?
   3. Where will you put your reservoir of water?
   4. How will you connect the pump's tubing from the reservoir of water to the plant? Will you need something to hold the tubing in place? Where will you put the outlet tube? At the edge of the pot or at the base of the plant? Somewhere in between?
   5. Where will you put the soil moisture sensor? Does it matter? For example, do you want to take readings near the base of the plant, or at the edge of the pot? If you put the sensor right next to the pump's outlet tube, what effect could this have?
   6. Do you need to do anything to protect the entire system — for example, from pets or small children?
4. Once your system is set up, you are ready to monitor its performance and your plant. Depending on how often you normally water your plant, this process could take days or weeks. Note that you might not be there to observe when the pump is running (you could be at school or asleep). Keep an eye on both the soil and the plant. Does the soil look too dried out or oversaturated? Does the plant look wilted? If so, you may need to adjust the wet and dry variables in your code.
5. Even when it is only powered on intermittently, the sensor's probes can oxidize over time. If you notice any drift in the system's performance, like a trend toward over- or under-watering, you may need to recalibrate your sensor. Your plant could also be growing and consuming more water. Continue to monitor the soil and your plant's health, and adjust thewet and dry variables as needed.
6. Can you get to a point where the automatic system can reliably water the plant for you? (You will still need to refill the external container of water occasionally.) Since this is an engineering design project, there are many other features you could add to your automatic watering system. See the Variations section for some ideas.