Documentation

// 5V 120L/H pump on MOSFET, gate on D9.
// Waters when the analog moisture reading is too dry.
// No sensor? Set USE_SENSOR to false for a simple timer.

const int  PUMP_PIN   = 9;
const int  SENSOR_PIN = A0;
const bool USE_SENSOR = true;
const int  DRY_LEVEL  = 600;   // calibrate for your sensor

void setup() {
  pinMode(PUMP_PIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  bool shouldWater = true;

  if (USE_SENSOR) {
    int m = analogRead(SENSOR_PIN);
    Serial.print("Moisture: ");
    Serial.println(m);
    shouldWater = (m > DRY_LEVEL);   // higher = drier for many sensors
  }

  if (shouldWater) {
    digitalWrite(PUMP_PIN, HIGH);
    delay(4000);                     // 4 s water shot
    digitalWrite(PUMP_PIN, LOW);
  }
  delay(60000);                      // check every minute
}

# Raspberry Pi controls the pump through a MOSFET or relay on GPIO 18.
# Pump power from the Pi's 5V pin (short runs) or an external 5V supply.

import RPi.GPIO as GPIO
import time

PUMP = 18
GPIO.setmode(GPIO.BCM)
GPIO.setup(PUMP, GPIO.OUT)

try:
    while True:
        print("Pump ON")
        GPIO.output(PUMP, GPIO.HIGH)
        time.sleep(5)          # circulate for 5 s
        print("Pump OFF")
        GPIO.output(PUMP, GPIO.LOW)
        time.sleep(55)         # rest for 55 s
except KeyboardInterrupt:
    pass
finally:
    GPIO.cleanup()