An optocoupler is a device that combines light-emitting and receiving devices in one package. Mostly it is a combination of the infrared light-emitting diode (LED) and a phototransistor. There are three main types of optocouplers:

• an optocoupler of a closed pair configuration is enclosed in the dark resin and is used to transfer signals using light. This type of optocoupler is not a sensor itself but is used for ensuring electrical isolation between two circuits;
• a slotted optocoupler has an open space between the light source and the sensor, light can be obstructed by external objects and thus can influence the sensor signal. It can be used to detect the presence of flat objects, measure rotation speed, vibrations or serve as a bounce-free switch;
• a reflective pair configuration the light signal is perceived as a reflection from the object surface. This configuration is used for proximity detection, surface colour detection and tachometer.

An example code:

int optoPin = A0; //Initialize an analogue A0 pin for optocoupler
int optoReading; //The analogue value reading from the optocoupler
 
int objecttreshold = 1000; //Object threshold definition
int whitetreshold = 150; //White colour threshold definition
 
void setup () 
{
  //Begin serial communication
  Serial.begin(9600); 
  //Initialize the analogue pin of the optocoupler as an input
  pinMode(optoPin, INPUT); 
}
 
void loop () 
{
  optoReading = analogRead(optoPin); //Read the value of the optocoupler
  Serial.print ("The reading of the optocoupler sensor is: ");
  Serial.println(optoReading);
 
  //When the reading value is lower than the object threshold
  if (optoReading < objecttreshold) { 
    Serial.println ("There is an object in front of the sensor!");
    //When the reading value is lower than the white threshold
    if (optoReading < white threshold) { 
      Serial.println ("Object is in white colour!");
    } else { //When the reading value is higher than the white threshold
      Serial.println ("Object is in dark colour!");
    }
  }
  else { //When the reading value is higher than the object threshold
    Serial.println ("There is no object in front of the sensor!");
  }
  delay(500); //Short delay
}

This type of sensor gives information about the colour of the light illuminating the sensor surface. Because computers often use RGB (red, green, blue) colour scheme sensor returns three values representing the intensity of three components. Colour sensors usually contain white LEDs to illuminate the surface which colour should be distinguished by them. The colour sensor uses SPI or TWI interface to send readings. Some models of colour sensors include an additional gesture detector which recognises simple gestures (up, down, left, right).

#include <Wire.h>
#include "Adafruit_TCS34725.h"
 
// Example code for the TCS34725 library by Adafruit
 
/* Connect SCL    to analog 5
   Connect SDA    to analog 4
   Connect VDD    to 3.3V DC
   Connect GROUND to common ground */
 
// Sensor class 
Adafruit_TCS34725 rgb_sensor = Adafruit_TCS34725();
 
void setup(void) {
  Serial.begin(9600);
 
  if (rgb_sensor.begin()) {  //Initialise RGB sensor
    Serial.println("RGB sensor present");
  } else {
    Serial.println("No TCS34725 found");
    while (1);
  }
}
 
void loop(void) {
  uint16_t r, g, b, unfiltered, lux;
 
  rgb_sensor.getRawData(&r, &g, &b, &unfiltered);   //read RGB and unfiltered light intensity
  lux = rgb_sensor.calculateLux(r, g, b);  //calculate illuminance in Lux
 
  Serial.print("Lux: "); 
  Serial.print(lux, DEC);
  Serial.print(" - ");
 
  Serial.print("R: "); 
  Serial.print(r, DEC); 
  Serial.print(" ");
 
  Serial.print("G: "); 
  Serial.print(g, DEC); 
  Serial.print(" ");
 
  Serial.print("B: "); 
  Serial.print(b, DEC); 
  Serial.print(" ");
 
  Serial.print("C: "); 
  Serial.print(unfiltered, DEC); 
  Serial.println(" ");
 
  delay(1000);
}