Sharp GP2Y0A21 IR Distance Sensor

1. Introduction

The Sharp GP2Y0A21 is an analog infrared distance sensor that uses triangulation to measure distance from 10cm to 80cm. It emits an IR beam and measures the angle of the reflected light to determine distance.

Unlike ultrasonic sensors, IR sensors work well on soft materials and are not affected by sound-absorbing surfaces. They're commonly used in robotics for obstacle detection.

2. Technical Specifications

How It Works

The sensor uses optical triangulation:

1. IR LED emits a beam of infrared light
2. Light reflects off the target surface
3. Position Sensitive Detector (PSD) senses the angle
4. Closer objects reflect light at a steeper angle
5. Sensor outputs voltage proportional to angle (non-linear)

3. Wiring Guide

The sensor has a 3-pin JST connector:

*ESP32: Use ADC1 pins (GPIO 32-39). Note ESP32 ADC is 12-bit (0-4095) and 3.3V reference.

Figure 1: Sharp GP2Y0A21 wiring with Arduino

4. Project: IR Distance Meter

Arduino Code (5V, 10-bit ADC)

const int sensorPin = A0;

void setup() {
    Serial.begin(115200);
    Serial.println("Sharp IR Distance Sensor (10-80cm)");
}

void loop() {
    int rawValue = analogRead(sensorPin);
    float voltage = rawValue * (5.0 / 1023.0);

    // Empirical formula for GP2Y0A21
    // Valid for distances > 10cm
    if (voltage > 0.4) {
        float distance = 27.86 * pow(voltage, -1.15);
        
        // Clamp to valid range
        if (distance < 10) distance = 10;
        if (distance > 80) distance = 80;
        
        Serial.print("Distance: ");
        Serial.print(distance, 1);
        Serial.println(" cm");
    } else {
        Serial.println("Out of range (>80cm)");
    }

    delay(100);
}

ESP32 Code (3.3V, 12-bit ADC)

const int sensorPin = 34;  // ADC1 pin

void setup() {
    Serial.begin(115200);
    analogReadResolution(12);  // Use full 12-bit resolution
}

void loop() {
    int rawValue = analogRead(sensorPin);
    // ESP32 is 3.3V reference, but sensor outputs up to 3.1V
    float voltage = rawValue * (3.3 / 4095.0);

    if (voltage > 0.3) {
        float distance = 27.86 * pow(voltage, -1.15);
        distance = constrain(distance, 10, 80);
        
        Serial.print("Distance: ");
        Serial.print(distance, 1);
        Serial.println(" cm");
    } else {
        Serial.println("Out of range");
    }

    delay(100);
}

5. Improving Accuracy

Averaging Multiple Readings

float getAverageDistance(int samples) {
    float total = 0;
    
    for (int i = 0; i < samples; i++) {
        int raw = analogRead(sensorPin);
        float voltage = raw * (5.0 / 1023.0);
        if (voltage > 0.4) {
            total += 27.86 * pow(voltage, -1.15);
        }
        delay(10);
    }
    
    return total / samples;
}

Lookup Table (More Accurate)

For best accuracy, create a lookup table by measuring actual distances and recording the corresponding ADC values. Interpolate between points.

6. Applications

• Obstacle detection: Robot collision avoidance
• Edge detection: Stair/cliff detection for robots
• Proximity sensing: Touchless switches
• Object counting: Simple break-beam detection

7. Troubleshooting

Readings are erratic

• Add 10-100µF capacitor across VCC and GND
• Average multiple readings

Incorrect distance values

• Verify voltage reference (5V vs 3.3V)
• Calibrate with known distances

Minimum distance issues

• Sensor cannot measure below 10cm accurately
• Objects too close give false high readings

8. Summary

• Analog IR distance sensor (10-80cm range)
• Requires 5V power supply
• Non-linear output: use formula or lookup table
• Formula: Distance = 27.86 × Voltage^(-1.15)
• Average readings for stability