GP2Y0A21YK Sharp IR Distance Sensor

Working Principle

The Sharp GP2Y0A21YK is an infrared triangulation distance sensor. An IR LED projects a spot of light onto a target. The reflected light hits a linear Position-Sensitive Detector (PSD). As the target distance changes, the reflected spot shifts along the PSD, changing the output voltage — a principle called optical triangulation.

Analogy: Hold your thumb at arm's length and close one eye, then the other — your thumb appears to shift (parallax). The GP2Y0A21YK uses this same principle with an IR LED and a detector instead of two eyes.

Electrical Characteristics

Parameter	Value
Supply Voltage	4.5 V – 5.5 V
Detection Range	10 – 80 cm
Output	Analog voltage (inversely proportional to distance)
Output Voltage	~0.4 V (80 cm) to ~3.1 V (10 cm)
Update Rate	~26 ms per measurement (38 Hz)
Current	~30 mA typical
Beam Width	Narrow (~5°)

Interfacing with an MCU

The sensor has a 3-pin JST connector: VCC (red), GND (black), and signal (yellow). Connect the signal wire to an ADC pin. A 10–47 µF bypass capacitor across VCC-GND reduces noise from the pulsed IR emitter.

Calibration

The output voltage is non-linear — inversely proportional to distance. Common linearisation methods:

• Empirical formula: distance_cm = 27.86 / (voltage - 0.1) (approximate fit for 10–80 cm).
• Lookup table: Measure voltage at 10, 20, 30, 40, 50, 60, 70, 80 cm; interpolate between samples.
• Averaging: Take 5–10 ADC reads and median-filter to reject spikes from noisy power supplies.

Code Example

/*
 * GP2Y0A21YK Sharp IR Distance — Arduino
 * Wiring: Signal→A0, VCC→5V, GND→GND
 * Add 10µF cap across VCC-GND
 */
#define SENSOR_PIN A0

float readDistance() {
    int total = 0;
    for (int i = 0; i < 10; i++) {
        total += analogRead(SENSOR_PIN);
        delay(2);
    }
    float voltage = (total / 10.0) * (5.0 / 1023.0);
    if (voltage < 0.15) return -1;  /* Out of range */
    return 27.86 / (voltage - 0.1);  /* Empirical formula */
}

void setup() {
    Serial.begin(9600);
    Serial.println("GP2Y0A21YK Ready");
}

void loop() {
    float dist = readDistance();
    if (dist > 0 && dist <= 80) {
        Serial.print("Distance: ");
        Serial.print(dist, 1);
        Serial.println(" cm");
    } else {
        Serial.println("Out of range");
    }
    delay(250);
}

Real-World Applications

Limitations

• Dead zone: Targets closer than 10 cm give incorrect (lower) voltage — mount sensors back from the chassis edge.
• Colour/surface dependence: Dark and absorptive surfaces reflect less IR, shortening the effective range.
• Sunlight interference: Outdoor IR noise can degrade accuracy; use an IR-pass filter or shade the sensor.
• Narrow view: The ~5° beam misses small obstacles between sensors; use multiple units or complement with ultrasonic.