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🌱 Thirsty-plant alert
“I want my phone to tell me when my houseplant needs water. Where do I start?”
A small WiFi board reads a soil-moisture sensor and pings your phone when the soil gets dry. One evening and about $10.
What to buy
| Part | Why this one | Rough cost |
|---|---|---|
| ESP32-C3 board (e.g. Seeed XIAO ESP32C3) | Small, cheap, WiFi built in. Plenty for reading one sensor. | $4–7 |
| Capacitive soil-moisture sensor | Measures moisture with no bare metal in the soil, so it doesn't corrode like two-prong probes. | $2–4 |
| USB cable and an old phone charger | The simplest power while you build. | you likely have one |
| 3 jumper wires | Sensor to board: power, ground and signal. | $1 |
Prices are rough ranges, not quotes. Check the current price and stock before you order.
What to install
Already run Home Assistant? Flash ESPHome onto the board. You describe the sensor in a short settings file (YAML) instead of writing code, and Home Assistant's phone app sends the alert.
No Home Assistant? A short Arduino sketch reads the sensor and sends a push message through ntfy, a free, open-source notification service: install the ntfy app, subscribe to a topic, and the board posts to that topic. Pick a long, hard-to-guess topic name, because anyone who knows it can read your alerts.
Build it in three steps
- Read the sensor. Wire the sensor's power to 3.3V, ground to GND, and its signal (AOUT) to an analog pin. On the ESP32-C3, use GPIO 0–4 (A0–A2 on the XIAO): its other analog pin doesn't work reliably while WiFi is on. Print the reading over USB and note two numbers: the sensor in dry air, and in a glass of water. Those are your dry and wet ends.
- Pick the alert point in real soil. Push the sensor into the pot up to the line marked on it, water the plant, and watch the number for a few days. Set the alert where the plant starts to look thirsty; every soil and pot is different.
- Send the alert. Check every 30–60 minutes, alert once when the soil passes your dry point, and don't alert again until it has been watered. Otherwise you get a message every half hour.
What catches people out
- Two-prong probes corrode. Bare metal prongs with current running through them oxidise in wet soil and the readings drift; they typically last a few months to a year (Adafruit, DFRobot). The capacitive type has no exposed metal, costs a dollar or two more and lasts much longer.
- Cheap capacitive boards vary. Some arrive with missing or wrong parts. Buy a spare and test both in air and in water before trusting them.
- Keep the top dry. Only the part below the line goes into the soil. Water on the electronics at the top ruins them; clear nail polish or heat-shrink on the edges helps.
- Battery later, not first. Get it working on USB. On a battery the board must sleep between readings, and reconnecting to WiFi each time is what drains it most.
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