YF-S201 Water Flow with an Arduino Uno
An inline Hall-effect water flow sensor that turns liquid flow into a pulse train, about 450 pulses per liter.
The YF-S201 is read by counting pulses, and on an Arduino Uno attachInterrupt() works only on D2 and D3, which is why DATA goes to D2 (INT0). Powered from the Uno's 5V, its pulses rise above 4.5 V, a clean high for the Uno with no divider.
Wiring the YF-S201 Water Flow to the Arduino Uno
Pin by pin.
| YF-S201 Water Flow | Arduino Uno |
|---|---|
| VCC | 5V |
| GND | GND |
| DATA | D2 |
What to know on the Arduino Uno
- The Uno has external interrupts on D2 (INT0) and D3 (INT1) only, so the flow sensor must use one of them; the second is free for another counter.Arduino language reference, attachInterrupt()
- At its 30 L/min maximum the sensor gives 7.5 × 30 = 225 pulses a second, an easy rate for the Uno's interrupt handler; the 1 s window then counts 225 pulses per 30 L/min.YF-S201 datasheet
- The Uno is an 8-bit chip, so reading the 4-byte pulses counter takes several instructions and an interrupt could change it halfway. That is why loop() copies it between noInterrupts() and interrupts().Arduino language reference, noInterrupts()
- Powered at 5 V its output rises above 4.5 V, a clean high for the Uno's 5 V inputs; the sensor draws 15 mA at most from the 5V pin.YF-S201C datasheet; YF-S201 datasheet
YF-S201 Water Flow code for the Arduino Uno
Compiled for arduino:avr:uno on 2026-10-29.
// YF-S201 Water Flow on Arduino Uno: wiring and a free simulation at https://www.tinkered.ai/components/yf-s201-water-flow-sensor/arduino-uno
#ifndef IRAM_ATTR // ESP32 puts the ISR in RAM so it survives a
#define IRAM_ATTR // flash access; AVR does not define the macro
#endif
volatile unsigned long pulses = 0;
void IRAM_ATTR onPulse() { pulses++; }
#define FLOW_PIN 2 // Uno: D2 = INT0. ESP32: any GPIO.
void setup() {
Serial.begin(115200);
pinMode(FLOW_PIN, INPUT_PULLUP);
attachInterrupt(digitalPinToInterrupt(FLOW_PIN), onPulse, RISING);
}
void loop() {
noInterrupts();
pulses = 0;
interrupts();
delay(1000); // count for 1 second
noInterrupts();
unsigned long n = pulses; // snapshot: the ISR may fire mid-read
interrupts();
float lpm = n / 7.5; // YF-S201: f(Hz) = 7.5 * Q(L/min)
Serial.print("Flow: ");
Serial.print(lpm);
Serial.println(" L/min");
}
YF-S201 Water Flow on the Arduino Uno: questions
Which Uno pin should the YF-S201 use?
D2 or D3, the Uno's only external interrupt pins. The example uses D2 with attachInterrupt(digitalPinToInterrupt(2), onPulse, RISING).
How do I convert YF-S201 pulses to litres per minute on the Uno?
Count pulses for one second and divide by 7.5: the sensor gives 7.5 Hz per L/min (YF-S201 datasheet). For volume, about 450 pulses make a litre.
Does the YF-S201 need a voltage divider on an Arduino Uno?
No. On 5V its pulses rise above 4.5 V, matching the Uno's logic. A divider is only needed for 3.3 V boards.
Keep going
This guide as plain Markdown. /components/yf-s201-water-flow-sensor/arduino-uno.md