Alert System with Photo Resistor, Buzzer and Laser (Concept)

What we want to achieve

Using an Arduino - we are using a Nano V3.0 compatible board - we want to demonstrate on how an alert system could be build, where an alert is triggered if an object interrupts the path of light.

Think of a customer entering a shop, an intruder entering an area.

You could reverse the example and trigger an alarm when the light path is restored - for example to detect if an object has been removed.


This tip uses a laser as a directed and focus light source. While we are only using a 650nm 5mW laser, please be always careful not to point the laser at persons, animals.

Our code can run in two modes

1) It will continuously sound alarm once it has been triggered. Only way to stop alert is reset the Arduino

2) It will sound alert when light path is lost and will stop sounding alert once light path is restored.


  • Arduino (compatible) development board - we used the Arduino Nano compatible board (see here)
  • Laser Transmitter Module - KY-008 (see here)
  • Passive Buzzer Module - KY-006 (see here)
  • Photo-resistor Module - KY-018 (see here
  • Arduino IDE (download here)


    • This tip is focused on using the Arduino IDE

    How to


       KY-006 Arduino
      Pin S D4
      Pin (middle) NOT USED
      Pin - GND


       KY-018 Arduino
      Pin S A0
      Pin + VCC
      Pin - GND


      Alert System with Laser, Photo-resistor and Buzzer


      // Photo Resistor Module KY-018 and Passive Buzzer Module KY-006 for Arduino
      // 20 January 2020
      // Author:
      // Hardware:
      // Arduino
      // Photo Resistor Module KY-018
      // Passive Buzzer Module KY-006
      // Wiring:
      // KY-018 -> Arduino
      // S -> A0
      // + (middle) -> VCC 3.3V or 5V
      // - (GND) -> GND
      // KY-006 -> Arduino
      // S -> D4
      // (middle) -> NOT USED
      // - (GND) -> GND
      // Purpose:
      // Sound alert when photo resistor is no longer getting light.
      // The code supports to modes controlled by "TriggerModeOnce"
      // triggerModeOnce = true; // will sound alert once light source is lost and will continue to sound even if light source is restorted (Intruder alert). To reset, push reset button
      // triggerModeOnce = false; // will sound alert only when light source is lost. When lught source is restored, sound stops (visitor entered shop)

      // Hardware config
      int photoModule = A0; // S Pin of Photo Resistor Module KY-021 on Arduino A0
      int buzzerModule = 4; // S Pin of Passive Buzzer Module KY-006 on Arduino D4

      // Software config
      boolean triggerModeOnce = true; // will sound alert once light source is lost and will continue to sound even if light source is restorted (Intruder alert). To reset, push reset button
      boolean photoTriggered = false; // indicator if photoresistor lost light source
      int triggerThreshold = 150; // threshold for alert trigger. Anything beyond this value coming from Photoresistor will trigger alert

      // setup PINs
      void setup() {
      pinMode(photoModule, INPUT);
      pinMode(buzzerModule, OUTPUT);


      void loop() {

      // sound alert if
      // 1) threshold is exceeded - or -
      // 2) triggerModeOnce is alert and has been triggered before
      if ((analogRead(photoModule) > triggerThreshold) || (photoTriggered && triggerModeOnce)) {
      photoTriggered = true; // mark trigger

      // reset buzzer
      digitalWrite(buzzerModule, HIGH);
      digitalWrite(buzzerModule, LOW);

      // sound alert with PWM
      for (int i = 0; i < 80; i++) { // make a sound
      digitalWrite(buzzerModule, HIGH); // send high signal to buzzer
      delay(1); // delay 1ms
      digitalWrite(buzzerModule, LOW); // send low signal to buzzer


      for (int j = 0; j < 100; j++) { //make another sound
      digitalWrite(buzzerModule, HIGH);
      delay(2); // delay 2ms
      digitalWrite(buzzerModule, LOW);


      } else {
      // make sure is down
      digitalWrite(buzzerModule, LOW);

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