Arduino - Light Sensor Triggers Relay

We are going to learn how to:

If the light sensor's analog value is lower than a threshold, turn relay on.
If the light sensor's analog value is greater than a threshold, turn relay off.

By connecting relay to light bulb, led strip, motor or actuator... We can use the light sensor to control light bulb, led strip, motor or actuator...

Hardware Required

1	×	Official Arduino Uno
1	×	Alternatively, DIYables ATMEGA328P Development Board
1	×	USB 2.0 cable type A/B (for USB-A PC)
1	×	USB 2.0 cable type C/B (for USB-C PC)
1	×	Light Sensor
1	×	10 kΩ Resistor
1	×	Relay
1	×	Breadboard
1	×	Jumper Wires
1	×	Optionally, Solenoid Lock
1	×	Optionally, 12V Power Adapter
1	×	Optionally, DC Power Jack
1	×	Recommended: Screw Terminal Block Shield for Arduino Uno
1	×	Recommended: Breadboard Shield for Arduino Uno
1	×	Recommended: Enclosure for Arduino Uno
1	×	Recommended: Prototyping Base Plate & Breadboard Kit for Arduino UNO

Or you can buy the following sensor kits:

1	×	DIYables Sensor Kit (30 sensors/displays)
1	×	DIYables Sensor Kit (18 sensors/displays)

Disclosure: Some links in this section are Amazon affiliate links. If you make a purchase through these links, we may earn a commission at no extra cost to you.
Additionally, some links direct to products from our own brand, DIYables .

The LDR light sensor is very affordable, but it requires a resistor for wiring, which can make the setup more complex. To simplify the wiring, you can use an LDR light sensor module as an alternative.

About Relay and Light Sensor

If you do not know about relay and light sensor (pinout, how it works, how to program ...), learn about them in the following tutorials:

Wiring Diagram

This image is created using Fritzing. Click to enlarge image

Arduino Code

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-light-sensor-triggers-relay */ // constants won't change const int LIGHT_SENSOR_PIN = A0; // Arduino pin connected to light sensor's pin const int RELAY_PIN = 3; // Arduino pin connected to Relay's pin const int ANALOG_THRESHOLD = 500; // variables will change: int analogValue; void setup() { pinMode(RELAY_PIN, OUTPUT); // set arduino pin to output mode } void loop() { analogValue = analogRead(LIGHT_SENSOR_PIN); // read the input on analog pin if(analogValue < ANALOG_THRESHOLD) digitalWrite(RELAY_PIN, HIGH); // turn on Relay else digitalWrite(RELAY_PIN, LOW); // turn off Relay }

Quick Steps

Connect Arduino to PC via USB cable
Open Arduino IDE, select the right board and port
Copy the above code and open with Arduino IDE
Click Upload button on Arduino IDE to upload code to Arduino

Radiates light to sensor
See the change of relay's state

Code Explanation

Read the line-by-line explanation in comment lines of source code!

Video Tutorial

We are considering to make the video tutorials. If you think the video tutorials are essential, please subscribe to our YouTube channel to give us motivation for making the videos.

Light Sensor and Relay on Commercial Products

Wemo Smart Plug, WiFi Enabled, Works with Alexa, Google Assistant & Apple HomeKit

The Best Arduino Starter Kit

See the best Arduino kit for beginner