Waterproof Ultrasonic Level Sensor: JSN-SR04T & A02YYUW

Working Principle

Ultrasonic level sensors (like the JSN-SR04T and A02YYUW) measure liquid level by emitting an ultrasonic pulse downward from above the liquid surface and timing the echo return. The distance to the surface is calculated from the time-of-flight: d = (t × v_sound) / 2. The liquid level is then: level = tank_height − d.

Difference from HC-SR04: These sensors use sealed, waterproof transducers (IP67/IP68) rated for humid and wet environments, with a single combined transmitter/receiver element. The HC-SR04 uses separate T/R elements and is not waterproof.

Electrical Characteristics

Parameter	JSN-SR04T	A02YYUW
Range	20–450 cm	3–450 cm
Resolution	~1 mm	~1 mm
Accuracy	±1 cm	±1 cm
Blind Zone	20 cm	3 cm
Supply Voltage	5 V (board) / 3.3–5 V (A02YYUW)
Interface	Trigger/Echo (HC-SR04 compatible)	UART (9600 baud) or PWM
Transducer Rating	IP67 (waterproof)	IP67
Cable Length	~2.5 m	~2.5 m

Interfacing with an MCU

JSN-SR04T: Uses the same trigger/echo protocol as HC-SR04 — send 10 µs trigger pulse, measure echo pulse width with pulseIn(). The waterproof transducer connects via a cable to the main board.

A02YYUW: Outputs distance in millimetres via UART (4 bytes: header, distance high, distance low, checksum) at 9600 baud. No trigger needed — it auto-measures continuously.

Calibration

• Empty tank distance: Measure distance when tank is empty; this is your maximum distance
• Full tank distance: Measure at full level; level = empty_distance − current_distance
• Temperature compensation: Speed of sound varies ~0.6 m/s per °C; for outdoor tanks, compensate using an ambient temperature sensor
• Averaging: Take median of 5 readings to reject spurious echoes from splashing or foam

Code Example

/*
 * JSN-SR04T Ultrasonic Level Sensor — Arduino
 * Wiring: Trig → Pin 9, Echo → Pin 10, VCC → 5V
 * Measures water level in a known-height tank
 */
#define TRIG_PIN 9
#define ECHO_PIN 10
#define TANK_HEIGHT_CM 100.0  /* Total tank height */

void setup() {
    Serial.begin(9600);
    pinMode(TRIG_PIN, OUTPUT);
    pinMode(ECHO_PIN, INPUT);
    Serial.println("Ultrasonic Level Sensor Ready");
}

float measureDistance() {
    /* Median of 5 readings */
    float readings[5];
    for (int i = 0; i < 5; i++) {
        digitalWrite(TRIG_PIN, LOW);
        delayMicroseconds(2);
        digitalWrite(TRIG_PIN, HIGH);
        delayMicroseconds(10);
        digitalWrite(TRIG_PIN, LOW);

        long dur = pulseIn(ECHO_PIN, HIGH, 30000);
        readings[i] = dur * 0.0343 / 2.0;
        delay(60);
    }
    /* Simple sort for median */
    for (int i = 0; i < 4; i++)
        for (int j = i+1; j < 5; j++)
            if (readings[j] < readings[i]) {
                float t = readings[i];
                readings[i] = readings[j];
                readings[j] = t;
            }
    return readings[2];  /* Median */
}

void loop() {
    float dist = measureDistance();
    float level = TANK_HEIGHT_CM - dist;
    float pct = (level / TANK_HEIGHT_CM) * 100.0;
    if (level < 0) level = 0;

    Serial.print("Distance: ");
    Serial.print(dist, 1);
    Serial.print(" cm  Level: ");
    Serial.print(level, 1);
    Serial.print(" cm  (");
    Serial.print(pct, 0);
    Serial.println("%)");

    delay(2000);
}

Real-World Applications

Limitations

• Foam and turbulence: Surface foam absorbs ultrasonic pulses; turbulent surfaces scatter echoes.
• Temperature sensitivity: Speed of sound changes ±0.17%/°C, affecting accuracy in uncompensated systems.
• Blind zone: Cannot measure within 3–20 cm of the transducer face (varies by model).
• Beam angle: The ~30° cone can reflect off tank walls in narrow tanks, producing false readings.
• Vapour and humidity: Dense vapor or condensation on the transducer face attenuates the signal.