Welcome to Project 40! Let’s have an honest moment here: keeping houseplants alive is surprisingly stressful. You either forget they exist for three weeks until they turn into crispy husks, or you overcompensate by waterboarding them every morning until they succumb to root rot.

Sure, we built an Automated Plant Waterer back in Project 9, but what if you just want a dashboard you can nervously check from the couch while eating Doritos? What if you want to know exactly how parched your fern is before you commit to getting up?

Enter the ESP32 Smart Plant Monitor. We are going to build a Wi-Fi connected sensor node that hosts its own local web server. No cloud subscriptions, no magic blue smoke, just pure, unadulterated data telling you that your Monstera is thirsty.

The Hardware: Ditching the Cheap Sensors

If you recall from Lesson 133: Listening to Plants, cheap resistive soil moisture sensors have a fatal flaw: electrolysis. You stick them in wet dirt, pass a current through them, and they literally dissolve into a rusty paste after a few weeks.

For a permanent dashboard setup, we are upgrading to Capacitive Soil Moisture Sensors. They measure moisture using capacitance rather than resistance, meaning their electronics are sealed away from the wet soil, preventing corrosion.

The Parts List

To build this, you’ll need:

• ESP32 NodeMCU Development Board - The brains and the Wi-Fi brawn.
• Capacitive Soil Moisture Sensor - Corrosion-resistant so it won’t disintegrate in your potting soil.
• Elegoo Breadboard Jumper Wires Kit - Because you can never have enough spaghetti wires.

Wiring It Up

The wiring is incredibly straightforward. The capacitive sensor outputs an analog voltage between 0V and 3.3V, which we can easily read with the ESP32’s built-in ADC (Analog-to-Digital Converter).

1. VCC on Sensor → 3V3 on ESP32
2. GND on Sensor → GND on ESP32
3. AOUT on Sensor → VP (GPIO36) on ESP32

Note: The ESP32 is a 3.3V logic device. Do not power the sensor from a 5V pin, or the analog output might exceed 3.3V and fry your ADC pin!

The Code: Hosting a Web Server

We are combining our knowledge from Lesson 153: Setting up a Local Web Server and Lesson 116: Analog Inputs.

The ESP32 will connect to your home Wi-Fi network and spin up an asynchronous web server. When you navigate to the ESP32’s IP address on your phone or computer, it will serve up an HTML page containing a beautiful gauge showing the moisture percentage.

// Snippet: Reading the Capacitive Sensor
const int sensorPin = 36; // VP pin

// You will need to calibrate these values for your specific sensor!
const int airValue = 3500;  // Value when sensor is in dry air
const int waterValue = 1500; // Value when sensor is submerged in water

void setup() {
  Serial.begin(115200);
}

void loop() {
  int sensorValue = analogRead(sensorPin);
  
  // Map the analog value to a percentage (0-100%)
  int moisturePercent = map(sensorValue, airValue, waterValue, 0, 100);
  
  // Constrain the percentage between 0 and 100 in case readings drift
  moisturePercent = constrain(moisturePercent, 0, 100);
  
  Serial.print("Moisture Level: ");
  Serial.print(moisturePercent);
  Serial.println("%");
  
  delay(2000);
}

Calibration is Key!

Capacitive sensors are not plug-and-play. Before you put it in the dirt, you must calibrate it.

1. Upload the code and open the Serial Monitor.
2. Hold the sensor in the dry air and record the value (this is your airValue).
3. Dip the sensor into a glass of water up to the white line and record the value (this is your waterValue).
4. Update those variables in your code.

Now, combine this reading logic with a basic Wi-Fi web server (like we did in Lesson 153), and you’ve got a live dashboard! Let your inner botanist rejoice.