Ultrasonic sensors are distance sensors that use sound waves to detect how far away an object is. They send out high frequency bursts of sound and listen for its echo. They then determine how far away the object is based on how long it takes for the sound to return to the sensor. This variety requires an Arduino library to operate.

NewPing Library

#include <NewPing.h>

NewPing mysensor(5, 6, 200);

void setup() {
  Serial.begin(9600);
}

void loop() {
  int pingTime = mysensor.ping();

  int distance = mysensor.ping_in();

  int distance_cm = mysensor.ping_cm();

  Serial.println(distance);
}

Flex sensors are resistance-based sensors that react to how much they get bent. The more they are bent, the larger the resistance across the sensor. The Arduino is able to measure the changing resistance to determine how much the sensor is bent.

void setup() {
  Serial.begin(9600);
}

void loop() {
  int val = analogRead(A0);

  Serial.println(val);
  delay(100);
}

Force sensors are sensors that change resistance based on how much force is applied on it. The more force applied, the lower the resistance. Force can be measured using the analog pins on the Arduino.

void setup() {
  Serial.begin(9600);
}

void loop() {
  int val = analogRead(A0);

  Serial.println(val);
}

Infrared distance sensors are sensors that measure how far away an object is by shooting infrared light at the object and measuring how much light is reflected back. They output an analog current which can be read by the Arduino.

void setup() {
  Serial.begin(9600);
}

void loop() {
  int val = analogRead(A0);

  Serial.println(val);

  delay(100);
}

PIR sensors (or passive infrared sensors) are motion sensors that measure subtle changes in infrared light in the room and register that change as movement. The processing is done on the chip and it outputs a current based on whether or not it detects motion.

int pin = 2;

void setup(){
  pinMode(pin, INPUT);

  pinMode(13, OUTPUT);
}

void loop(){
  int pirVal = digitalRead(pin);

  if (pirVal == 0) {
    digitalWrite(13, HIGH);
  } else {
    digitalWrite(13, LOW);
  }

  delay(100);
}