Hall-Effect Water Flow Sensor: YF-S201

Working Principle

The YF-S201 is a Hall-effect turbine flow sensor used to measure liquid (water) flow rate. Inside the sensor body, water flows through a chamber containing a small pinwheel rotor with embedded magnets. As water spins the rotor, the magnets pass a Hall-effect sensor mounted on the outside of the chamber, generating a square-wave pulse output. The pulse frequency is proportional to the flow rate.

Conversion: The YF-S201 outputs approximately 7.5 pulses per litre (at its rated flow range). By counting pulses per second and dividing by the calibration factor, you get litres per minute. Integrating over time gives total volume.

Electrical Characteristics

Parameter	Value
Flow Range	1–30 L/min
Pulse Output	~7.5 pulses/litre (F = 7.5 × Q)
Supply Voltage	5–18 V DC
Output Type	Open-collector square wave (needs pull-up)
Max Water Pressure	1.75 MPa
Accuracy	±10% (typical, improvable with calibration)
Pipe Size	1/2″ (DN15) male threads
Operating Temperature	−25 to +80 °C

Interfacing with an MCU

Connect VCC (5V or higher), GND, and signal output to an interrupt-capable MCU pin. A 10 kΩ pull-up resistor is required on the signal line (some breakout boards include it). Use hardware interrupts for accurate pulse counting.

Calibration

• Factory factor: 7.5 pulses/litre (approximate); actual factor varies ±10% between units
• Per-unit calibration: Flow a known volume (e.g., 1 L from a measuring cup) and count total pulses; your actual factor = pulses / litres
• Flow-dependent factor: The calibration factor may vary slightly across the flow range; calibrate at your typical operating point

Code Example

/*
 * YF-S201 Water Flow Sensor — Arduino
 * Wiring: Red → 5V, Black → GND, Yellow → Pin 2
 *         10k pull-up on signal pin
 */
#define FLOW_PIN 2
#define CAL_FACTOR 7.5   /* Pulses per litre (tune per unit) */

volatile unsigned long pulseCount = 0;

void flowISR() {
    pulseCount++;
}

void setup() {
    Serial.begin(9600);
    pinMode(FLOW_PIN, INPUT_PULLUP);
    attachInterrupt(digitalPinToInterrupt(FLOW_PIN),
                    flowISR, RISING);
    Serial.println("YF-S201 Flow Sensor Ready");
}

void loop() {
    static unsigned long totalPulses = 0;
    static unsigned long lastTime = 0;

    if (millis() - lastTime >= 1000) {
        lastTime = millis();
        noInterrupts();
        unsigned long count = pulseCount;
        pulseCount = 0;
        interrupts();

        totalPulses += count;
        float flowRate = count / CAL_FACTOR;   /* L/min */
        float totalL  = totalPulses / CAL_FACTOR;

        Serial.print("Flow: ");
        Serial.print(flowRate, 2);
        Serial.print(" L/min  Total: ");
        Serial.print(totalL, 1);
        Serial.println(" L");
    }
}

Real-World Applications

Limitations

• Low flow dead zone: Below ~1 L/min, the rotor may not spin; not suitable for drip-rate measurement.
• Water only: Not rated for corrosive liquids, oils, or fluids with suspended solids (clogs the turbine).
• Accuracy: ±10% out of box; requires per-unit calibration for better than ±5%.
• Pressure drop: The turbine introduces a small pressure drop (~0.02 MPa at max flow).
• Wear: The plastic rotor and bearings wear over time, changing the calibration factor.