You need to know if an object is close to another one but without using a contact switch? The TCRT5000 sensor is a great and cheap option for that! It essentially consists into 2 parts bundled together: an infrared emitter and a photo-transistor.

 

Drawing 1: No object close to sensor
Drawing 2: An object is close to the sensor, so the infrared light is reflected to the photo-transistor.

You can’t really use that sensor to measure distance since it can only answer one question: Is an object close to the sensor or not? Well, actually, it could be used to measure distances between 2mm and 1.5cm or so.

TRCT5000 Breakout

Creating a breakout for the TRCT5000 is very simple. You simply implement this schematic:

The corresponding board looks like this:

Wiring the breakout to your Arduino

Programming your Arduino

To use this sensor, you simply call analogRead() as is:

#define LED_PIN 6
#define TCRT5000_SENSOR_PIN A2

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int reading = analogRead(TCRT5000_SENSOR_PIN);  
  // read is 0..1024, write is 0..255
  int reading = reading / 4;
  analogWrite(LED_PIN, reading);
  Serial.println(reading);
}