A Smart Auto-Watering System with Arduino

Auto-watering is the second “must-try” DIY project after the weather station: cheap, easy to wire, and it solves a real problem — forgetting to water your plants while you’re away.

How it works

1. A soil moisture sensor reads an analog value.
2. The Arduino compares it against a preset threshold.
3. If the soil is dry, the Arduino switches a relay to run a mini pump for a few seconds, then turns it off.

Parts

• Arduino Uno (or any board with an analog pin)
• A soil moisture sensor (capacitive type — more durable than resistive)
• A 1-channel relay module
• A 5V or 12V mini water pump + small tubing
• A separate power supply for the pump (don’t share the Arduino’s USB power)

Sample code

const int SOIL_PIN = A0;
const int RELAY_PIN = 7;
const int DRY_THRESHOLD = 500; // calibrate for your own sensor
const unsigned long WATER_DURATION = 4000; // 4 seconds per watering
const unsigned long CHECK_INTERVAL = 3600000UL; // check every hour

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, HIGH); // most relay modules are active LOW
  Serial.begin(9600);
}

void loop() {
  int moisture = analogRead(SOIL_PIN);
  Serial.println(moisture);

  if (moisture > DRY_THRESHOLD) {
    digitalWrite(RELAY_PIN, LOW);
    delay(WATER_DURATION);
    digitalWrite(RELAY_PIN, HIGH);
  }

  delay(CHECK_INTERVAL);
}

Note: DRY_THRESHOLD depends entirely on your specific sensor and soil — print the raw value to the Serial Monitor both when the soil is bone dry and right after watering to find the right threshold.

Common pitfalls

• Resistive sensors corrode after a few weeks sitting in damp soil — prefer capacitive sensors, or only power the sensor while taking a reading.
• Not isolating pump power from the Arduino’s power can cause noise or reset the board when the pump kicks on.
• Long delay() calls (like CHECK_INTERVAL) block the whole program — if you later add an LCD or a button, switch to a millis()-based timer instead.

Where to go from here

Pair it with an ESP32 and push moisture data into Home Assistant (see the ESP32 weather station post) for remote monitoring, or add a small solar panel to run it independently on a balcony.