ML8511: UV Index Sensor (UV-A/UV-B Photodiode)

Working Principle

The ML8511 from LAPIS Semiconductor (Rohm) is a UV-A/UV-B photodiode sensor with a built-in amplifier that outputs an analog voltage proportional to the UV light intensity (280–390 nm). The internal photodiode has a spectral response tuned to the UV region, and the on-chip amplifier provides a linear voltage output from ~1.0 V (no UV) to ~2.8 V (strong UV, ~15 mW/cm²).

UV intensity is commonly expressed as a UV Index (0–11+), which is a standardised measure of the erythema-weighted UV irradiance. The ML8511 output can be mapped to approximate UV Index for health-related applications (sunburn risk assessment).

Electrical Characteristics

Parameter	Value
Spectral Range	280–390 nm (UV-A + UV-B)
Output Voltage	~1.0 V (no UV) to ~2.8 V (15 mW/cm²)
Peak Sensitivity	~365 nm (UV-A)
Supply Voltage	2.7–3.6 V
Current Draw	~300 µA active, ~0.1 µA standby
Output	Analog voltage (linear with UV intensity)
Response Time	<0.5 seconds
Enable Pin	Active HIGH (pulls low for shutdown)

Interfacing with an MCU

Connect VCC (3.3 V), GND, EN (enable, tie to 3.3 V for always-on or control via GPIO), and OUT to an ADC pin. For best accuracy, use the 3.3 V rail as your ADC reference voltage. The SparkFun ML8511 breakout provides a 3.3 V reference pin for ratiometric measurements.

Calibration

• Two-point mapping: At ~1.0 V output = 0 mW/cm²; at ~2.2 V = ~10 mW/cm². Interpolate linearly between these points
• UV Index conversion: UV Index ≈ UV intensity (mW/cm²) / 0.25 (approximate, varies by weighting standard)
• Cross-reference: Compare readings against a calibrated UV meter or weather service UV Index for your location

Code Example

/*
 * ML8511 UV Index Sensor — Arduino (3.3V board or level-shifted)
 * SparkFun breakout: OUT → A0, 3V3 ref → A1, EN → 3.3V
 */
#define UV_PIN  A0
#define REF_PIN A1  /* 3.3V reference for ratiometric reading */

float readUV() {
    int uvRaw  = analogRead(UV_PIN);
    int refRaw = analogRead(REF_PIN);

    /* Ratiometric conversion */
    float voltage = 3.3 * (float)uvRaw / (float)refRaw;
    return voltage;
}

float uvToIndex(float voltage) {
    /* Linear mapping: 1.0V = 0 mW/cm², 2.2V = 10 mW/cm² */
    float intensity = max(0.0, (voltage - 1.0) / (2.2 - 1.0) * 10.0);
    float uvIndex = intensity / 0.25;  /* Approximate UV Index */
    return uvIndex;
}

void setup() {
    Serial.begin(9600);
    Serial.println("ML8511 UV Sensor Ready");
}

void loop() {
    float uv = readUV();
    float idx = uvToIndex(uv);

    Serial.print("UV Voltage: ");
    Serial.print(uv, 2);
    Serial.print(" V  UV Index: ");
    Serial.print(idx, 1);

    if (idx < 3)       Serial.println("  (Low)");
    else if (idx < 6)  Serial.println("  (Moderate)");
    else if (idx < 8)  Serial.println("  (High)");
    else if (idx < 11) Serial.println("  (Very High)");
    else                Serial.println("  (EXTREME)");

    delay(2000);
}

Real-World Applications

Limitations

• Approximate UV Index: Not calibrated to the CIE erythema action spectrum; UV Index is approximate.
• No UV-C: Spectral response cuts off below 280 nm; cannot measure UV-C (germicidal wavelengths).
• Temperature drift: Output voltage drifts slightly with temperature; compensate for precision applications.
• Indoor limitation: UV levels indoors are very low; sensor is primarily for outdoor use.