QRE1113 Digital Sensor Overview

The onboard QRE1113 contains a 940nm infrared emitter beside an infrared-sensitive phototransistor. The module circuit includes an LED current-limiting resistor and a small timing capacitor so the amount of reflected infrared light can be measured using a digital I/O pin.

Important – This Is a Timed Digital Output

The normal measurement sequence is:

1. Configure the OUT connection as a digital output.
2. Drive it HIGH briefly to prepare the timing circuit.
3. Change the same GPIO to an input without a pull-up.
4. Measure the time until the input becomes LOW.
5. Interpret shorter times as more reflected infrared light and longer times as less reflection.

This arrangement provides a useful relative reflectance measurement without using an analogue input.

Connections & Arduino UNO Wiring

The three connections are printed on the module. Follow the VCC, OUT and GND markings rather than relying on connector orientation.

Mounting the Sensor

Point the QRE1113 sensing face directly towards the surface. Start with the sensor approximately 3mm from the floor or target and adjust the height only after checking the readings on the actual material.

For cliff detection, position the sensor far enough ahead of the wheels that the controller has time to stop or turn before the robot reaches the edge. Faster robots require more physical and software margin.

Arduino UNO Basic Reflectance Test

No external Arduino library is required. This example uses standard Arduino digital I/O and the micros() timer.

const byte SENSOR_PIN = 2;
const unsigned long TIMEOUT_US = 3000;

unsigned long readQRE1113() {
  // Drive the timing node HIGH briefly.
  pinMode(SENSOR_PIN, OUTPUT);
  digitalWrite(SENSOR_PIN, HIGH);
  delayMicroseconds(10);

  // Release the pin and measure how quickly it falls LOW.
  pinMode(SENSOR_PIN, INPUT);

  unsigned long startTime = micros();

  while (digitalRead(SENSOR_PIN) == HIGH) {
    unsigned long elapsed = micros() - startTime;

    if (elapsed >= TIMEOUT_US) {
      return TIMEOUT_US;
    }
  }

  return micros() - startTime;
}

void setup() {
  Serial.begin(115200);
  Serial.println("QRE1113 digital reflectance test");
  Serial.println("Lower value = more reflected IR");
}

void loop() {
  unsigned long reading = readQRE1113();

  Serial.print("Discharge time: ");
  Serial.print(reading);
  Serial.println(" us");

  delay(100);
}

Expected Result

Open the Serial Monitor at 115200 baud. You should see a changing discharge time in microseconds.

Do not expect a universal number for black, white or a cliff. The useful measurement is the difference between the surfaces in your own installation.

Calibrating Line and Cliff Detection

1. Mount the Sensor First

Calibration should be performed with the sensor at its final height and angle. Moving the sensor after calibration changes the amount of reflected infrared light and can move the threshold significantly.

2. Record the Real Readings

Run the basic test and record several readings for each condition the robot must distinguish. For example:

• normal floor;
• black or coloured tracking line;
• floor edge or open space;
• any different floor materials the robot may encounter.

3. Choose a Threshold Between the Groups

If the normal floor produces much shorter readings than the open edge, choose a cliff threshold between the highest normal-floor reading and the lowest reliable edge reading. Leave as much margin as practical rather than choosing a threshold close to either group.

For a dark line on a lighter floor, the dark line will commonly produce the longer timing value. Your control program can therefore compare the measured time with a calibrated threshold to decide whether the sensor is over the floor or line.

4. Test Under Real Lighting

Repeat the test under the lighting in which the robot will operate. Strong sunlight and other infrared sources can alter the phototransistor response. Mechanical vibration, floor texture and sensor angle can also increase variation.

Using More Than One Sensor

Multiple modules can be fitted across the front of a robot for wider line or edge coverage. Each digital capacitor-discharge module requires its own bidirectional GPIO. The simple timing method reads one sensor at a time, so allow for the total measurement time when several sensors are used.

Troubleshooting