ESP32 Plant Monitoring Learning Kit

Build a USB-powered ESP32 learning prototype that calibrates a capacitive soil sensor and a short-duration water sensor, reads a 3.3 V BME280, and reports local Serial Monitor data. The checked firmware includes guided calibration and fault handling; no pump, Wi-Fi dashboard, unattended control, or verified buzzer driver is included.

Each project has a table with three columns: the part and its pin, where it connects on the board (the label printed next to the pin), and a note. “through 220 Ω” means a resistor sits between the board pin and the part. GND is ground — all GND pins and rails are connected together.

LEDs have a long leg (+, anode) and a short leg (−, cathode). Resistor values are coded in colours: 220 Ω is red-red-brown, 1 kΩ brown-black-red, 10 kΩ brown-black-orange.

The capacitive soil sensor gives a voltage that changes as the soil around it gets wetter; the water sensor's exposed tracks conduct more the deeper they sit in water. The ESP32 reads each as a number from 0 to 4095, averaging 24 readings to smooth out noise.

Those raw numbers mean nothing on their own, so you teach the sketch four reference points: soil-dry, soil-wet, water-empty and water-full. It needs at least 150 counts between each pair, and saves them in flash so you only do this once.

In monitoring mode it converts each raw reading into a percentage between your two reference points. Soil under 30 % adds DRY_SOIL to the status; water under 20 % adds LOW_WATER. These are teaching defaults, not advice for any particular plant.

The BME280 adds temperature, humidity and pressure. If it fails three reads in a row, the sketch reports a fault instead of printing wrong numbers.

Every reading is printed as one DATA line with name=value pairs, easy to read and easy to import into a spreadsheet.

1. In Arduino IDE install the ESP32 boards package (Boards Manager → esp32 by Espressif) and, from Library Manager, Adafruit BME280 and Adafruit Unified Sensor. Select the ESP32 Dev Module board and your USB port.
2. Plug in the ESP32 alone and upload any example sketch to prove the cable and port work.
3. With USB unplugged, fit the ESP32 across the breadboard centre gap (or beside it with female-to-male jumpers if it is too wide). Link 3V3 to the + rail and GND to the − rail. Check whether the long rails are split in the middle.
4. Add the BME280 first: VIN to 3V3, GND, SDA to GPIO21, SCL to GPIO22. Upload the main sketch. Open Tools → Serial Monitor at 115200 baud, with the line ending set to Newline. It should report one BME280 at 0x76 or 0x77.
5. Unplug USB. Add the soil sensor: VCC to 3V3, GND, AOUT to GPIO34. Plug back in and check the CAL lines show soil_raw changing when you touch the sensor's blade — not stuck at 0 or 4095.
6. Unplug USB. Add the water sensor last: + to 3V3, − to GND, S to GPIO35. Keep it dry for now.
7. Calibrate the soil: hold the probe in air (or truly dry soil) and type soil-dry. Then push it into wet soil up to the line on the board and type soil-wet.
8. Calibrate the water: with the sensor dry type water-empty. Lower only the tracks into the water to a depth you mark, and type water-full. Lift it out, rinse and dry it.
9. Type status. All four values must be present with each pair at least 150 apart. Type run to start monitoring.
10. Push the soil probe into a plant pot and watch soil_pct. Water the plant and see it rise; let a test sample dry over a day and see it fall.

The sensor's blade is one plate of a capacitor; the soil around it is the material in between. Water changes how much charge the capacitor holds, and the module turns that into a voltage. The ESP32 reads the voltage as a number from 0 to 4095 — on most of these sensors the number goes down as the soil gets wetter.

Type d with the sensor in air and the sketch saves the dry end; type w with it in a glass of water and it saves the wet end. Every reading after that becomes a percentage between those two ends.

That is why two kits never agree on '40 %': the percentage depends on the soil type, how deep the probe sits, how tightly the soil is packed and the endpoints you chose. The sketch also keeps the lowest and highest raw values it has seen so you can judge how stable a reading is.

1. In Arduino IDE install the ESP32 boards package (Boards Manager → esp32 by Espressif) and, from Library Manager, Adafruit BME280 and Adafruit Unified Sensor. Select the ESP32 Dev Module board and your USB port.
2. With USB unplugged, wire the soil sensor: VCC to 3V3, GND to GND, AOUT to GPIO34.
3. Upload the Soil-moisture calibration study sketch. Open Tools → Serial Monitor at 115200 baud, with the line ending set to Newline.
4. Hold the probe in the air, wait for the raw value to settle, and type d. Then dip the blade in a glass of water up to the line (never over the electronics) and type w.
5. Push the probe into the dry, damp and wet soil cups in turn, to the same depth. Wait 10 seconds in each and write down raw and percent.
6. Repeat in the damp cup at half depth, and again with the soil pressed down firmly. Note how much the reading moves.
7. Plot raw value against cup in a spreadsheet. Type r to reset the endpoints and try calibrating with dry and wet soil instead of air and water.

The board has interleaved copper tracks. Water bridges them, and the deeper it goes the more track is bridged, so the voltage on S — and the ESP32's raw reading — rises with depth.

The sketch remembers the lowest raw value it has seen as 'empty' and the highest as 'full', and reports the level as a percentage between them. Below 20 % it prints EVENT: LOW WATER once; rising back above, EVENT: water OK.

The response is not linear, and the tracks slowly corrode in water, so readings drift over time. That is why this sensor suits short supervised tests, not a permanent tank gauge.

1. In Arduino IDE install the ESP32 boards package (Boards Manager → esp32 by Espressif) and, from Library Manager, Adafruit BME280 and Adafruit Unified Sensor. Select the ESP32 Dev Module board and your USB port.
2. With USB unplugged, wire the water sensor: + to 3V3, − to GND, S to GPIO35. Use longer jumpers so the electronics stay well away from the water.
3. Mark depths on the sensor with a pen: 0.5, 1, 2, 3 cm from the bottom of the tracks.
4. Upload the Short-duration water-level experiment sketch. Open Tools → Serial Monitor at 115200 baud, with the line ending set to Newline. Start with the sensor dry.
5. Lower it to each mark and hold it still for a few seconds. Write down raw and level_pct at each depth.
6. Lift it back out step by step and note whether the readings match on the way up. Watch for the LOW WATER and water OK events.
7. Unplug USB, rinse the tracks in clean water and dry them with a paper towel.
8. Plot raw against depth. Is it a straight line? Repeat tomorrow and compare.

The BME280 measures temperature, relative humidity and barometric pressure inside one chip and sends the results over the I2C bus (two wires, SDA and SCL).

Pressure falls as you go up and changes with the weather. Sea level on an average day is about 1013 hPa; the sketch uses that to estimate altitude, so the altitude figure moves when the weather changes even if you do not.

Each row also keeps the lowest and highest temperature since start-up. Breathe gently near the sensor and watch humidity jump and slowly settle — that settling time is part of what you are measuring.

1. In Arduino IDE install the ESP32 boards package (Boards Manager → esp32 by Espressif) and, from Library Manager, Adafruit BME280 and Adafruit Unified Sensor. Select the ESP32 Dev Module board and your USB port.
2. With USB unplugged, wire the BME280: VIN to 3V3, GND, SDA to GPIO21, SCL to GPIO22. Place it at the far end of the breadboard, away from the ESP32, which gets warm.
3. Upload the BME280 room environment station sketch. Open Tools → Serial Monitor at 115200 baud, with the line ending set to Newline.
4. Leave it for 5 minutes and note where temperature and humidity settle.
5. Breathe gently towards the sensor once. Note how high humidity goes and how many seconds it takes to come back.
6. Move the board near a window, a fan or a plant and compare. Keep it out of direct sun and away from water.
7. Compare the pressure with a local weather report for your city.

Every LOG_EVERY_S seconds the sketch prints one comma-separated row: row number, seconds since start, soil raw, water raw, temperature, humidity and pressure. If the BME280 is missing it prints NA in those columns instead of stopping.

Raw soil and water values are logged on purpose. You can convert them to percentages later in the spreadsheet using your dry/wet endpoints, without running the experiment again.

The kit has no clock chip, so time is 'seconds since start'. Note the real start time and add it in the spreadsheet.

1. Build the main project wiring (or keep it from the main build): BME280 on GPIO21/22, soil on GPIO34, water on GPIO35.
2. For a quick test leave LOG_EVERY_S at 5. For a real afternoon, change it to 300 (one row every 5 minutes).
3. Upload the Serial data logger and charts sketch. Open Tools → Serial Monitor at 115200 baud, with the line ending set to Newline. Write down the clock time when it starts.
4. Push the soil probe into a pot of dry-ish soil and leave the computer running. Keep the water sensor out of water unless you are doing a short test.
5. After a few rows, water the plant and keep logging for an hour or more.
6. Untick 'Show timestamp' in the Serial Monitor, then copy all the text and paste it into a spreadsheet. Use Data → Split text to columns (comma) if it lands in one column.
7. Make a line chart of soil_raw against t_s, and another of temperature and humidity.

Optional DIY-016 5 V buzzer, ESP32U, and additional user-supplied driver/protection parts selected after measurement

Completed local main build and user-supplied Wi-Fi network

Every project in this kit uses the same parts, so you can combine them: add the display to a sensor project, or a buzzer to a measuring one. The project pages on diykit.ae have the code, the parts list and ideas to take each build further.

Stuck on a wire, a library or an error message? Send us a photo of your wiring and the error text on WhatsApp — we answer build questions.