YF-S201 flow sensor with ESP32 DevKit V1 — litres and L/min

The YF-S201 is a small turbine in a ½-inch pipe section: water spins a magnet past a Hall sensor, which gives one pulse per turn. Count the pulses and you get flow in litres per minute and the total volume — useful for tank-filling monitors, borewell pump logs and water-bill checks.

On the ESP32 DevKit V1 it needs 5 V from VIN to run, and a 10 kΩ / 20 kΩ divider on the pulse line so the GPIO only ever sees about 3.3 V. The sketch counts pulses in an interrupt and prints flow and total every second.

Wiring

Board quirks

• Why the divider: powered from 5 V, the pulse output can swing close to 5 V, which an ESP32 pin must not see. We assume the common case of an output that pulls up towards VCC inside the sensor; the divider makes it safe either way. If pulses go missing, measure SIG’s HIGH level at D27 with a meter — it should be 2.5–3.3 V.
• Counting happens in an IRAM_ATTR interrupt on the falling edge, so no pulse is lost while the ESP32 is busy with Wi-Fi. At full flow (30 L/min) that is only about 225 pulses a second.
• The 7.5 and 450 figures are typical, not exact. Fill a 10-litre bucket, read the pulse total, and set PULSES_PER_LITRE = pulses ÷ 10 for your sensor and pipe.
• Below about 1 L/min the rotor stalls and reads zero — a slowly dripping tap won’t register. Mount the sensor with the arrow along the flow and the pipe full of water, not upside down on a vertical pipe.
• For overhead-tank monitoring the ESP32 can keep the total across resets in Preferences (flash), and power cuts won’t wipe it.

Code

// YF-S201 water flow sensor on an ESP32 DevKit V1.
// VCC (red) → VIN (5 V), GND (black) → GND,
// SIG (yellow) → 10 kΩ → D27 (GPIO27), with 20 kΩ from D27 to GND (5 V pulses → about 3.3 V).
// About 7.5 pulses per second per L/min (≈ 450 pulses per litre) — calibrate for your sensor.

constexpr uint8_t FLOW_PIN = 27;
constexpr float PULSES_PER_LITRE = 450.0f;

volatile uint32_t pulses = 0;

void IRAM_ATTR onPulse() { pulses++; }

void setup() {
  Serial.begin(115200);
  pinMode(FLOW_PIN, INPUT);  // the divider sets the level; no internal pull
  attachInterrupt(digitalPinToInterrupt(FLOW_PIN), onPulse, FALLING);
  Serial.println("YF-S201 on GPIO27 — open the tap");
}

void loop() {
  static uint32_t lastTotal = 0;
  delay(1000);
  noInterrupts();
  uint32_t total = pulses;
  interrupts();

  uint32_t perSecond = total - lastTotal;
  lastTotal = total;
  float litresPerMin = perSecond / 7.5f;
  float litres = total / PULSES_PER_LITRE;
  Serial.printf("Flow %.2f L/min  |  Total %.2f L  (%lu pulses)\n", litresPerMin, litres, (unsigned long)total);
}

// YF-S201 water flow sensor on an ESP32 DevKit V1.
// VCC (red) → VIN (5 V), GND (black) → GND,
// SIG (yellow) → 10 kΩ → D27 (GPIO27), with 20 kΩ from D27 to GND (5 V pulses → about 3.3 V).
// About 7.5 pulses per second per L/min (≈ 450 pulses per litre) — calibrate for your sensor.

constexpr uint8_t FLOW_PIN = 27;
constexpr float PULSES_PER_LITRE = 450.0f;

volatile uint32_t pulses = 0;

void IRAM_ATTR onPulse() { pulses++; }

void setup() {
  Serial.begin(115200);
  pinMode(FLOW_PIN, INPUT);  // the divider sets the level; no internal pull
  attachInterrupt(digitalPinToInterrupt(FLOW_PIN), onPulse, FALLING);
  Serial.println("YF-S201 on GPIO27 — open the tap");
}

void loop() {
  static uint32_t lastTotal = 0;
  delay(1000);
  noInterrupts();
  uint32_t total = pulses;
  interrupts();

  uint32_t perSecond = total - lastTotal;
  lastTotal = total;
  float litresPerMin = perSecond / 7.5f;
  float litres = total / PULSES_PER_LITRE;
  Serial.printf("Flow %.2f L/min  |  Total %.2f L  (%lu pulses)\n", litresPerMin, litres, (unsigned long)total);
}

Expected serial output

YF-S201 on GPIO27 — open the tap
Flow 0.00 L/min  |  Total 0.00 L  (0 pulses)
Flow 6.27 L/min  |  Total 0.10 L  (47 pulses)
Flow 8.40 L/min  |  Total 0.24 L  (110 pulses)
Flow 8.53 L/min  |  Total 0.39 L  (174 pulses)

YF-S201 on GPIO27 — open the tap
Flow 0.00 L/min  |  Total 0.00 L  (0 pulses)
Flow 6.27 L/min  |  Total 0.10 L  (47 pulses)
Flow 8.40 L/min  |  Total 0.24 L  (110 pulses)
Flow 8.53 L/min  |  Total 0.39 L  (174 pulses)

Common errors