Arduino Nano - Light Sensor Relay

This tutorial instructs you how to use Arduino Nano and light sensor to trigger relay. In detail:

Arduino Nano activates the relay if the analog value of the light sensor is less than a certain threshold.
Arduino Nano deactivates the relay if the analog value of the light sensor is greater than a certain threshold.

By connecting a relay to a light bulb, LED strip, motor or actuator, we can use the Arduino Nano and light sensor to manage the operation of the light bulb, LED strip, motor or actuator.

The light sensor is also known as photoresistor, light-dependent resistor, photocell, LDR. The Arduino Nano utilizes a light sensor to measure the ambient light level, and it activates the relay when it's dark, and deactivates it when it's bright.

Hardware Preparation

1	×	Official Arduino Nano
1	×	Alternatively, DIYables Nano V3.0 Development Board
1	×	USB A to Mini-B USB cable
1	×	Light Sensor
1	×	10 kΩ Resistor
1	×	Relay
1	×	Warning Light Bright Waterproof
1	×	12V Power Adapter
1	×	DC Power Jack
1	×	Breadboard
1	×	Jumper Wires
1	×	Recommended: Screw Terminal Expansion Board for Arduino Nano
1	×	Recommended: Breakout Expansion Board for Arduino Nano
1	×	Recommended: Power Splitter for Arduino Nano

Or you can buy the following kits:

DIYables Sensor Kit (18 sensors/displays)

Disclosure: Some of the links provided in this section are Amazon affiliate links. We may receive a commission for any purchases made through these links at no additional cost to you.
Additionally, some of these links are for 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.

Overview of Relay and Light Sensor

If you are unfamiliar with relay and light sensor (including pinout, how it works, and programming), the following tutorials can help:

Wiring Diagram

This image is created using Fritzing. Click to enlarge image

See The best way to supply power to the Arduino Nano and other components.

Arduino Nano Code

/* * This Arduino Nano code was developed by newbiely.com * * This Arduino Nano code is made available for public use without any restriction * * For comprehensive instructions and wiring diagrams, please visit: * https://newbiely.com/tutorials/arduino-nano/arduino-nano-light-sensor-relay */ const int LIGHT_SENSOR_PIN = A0; // The Arduino Nano pin connected to light sensor's pin const int RELAY_PIN = 2; // The Arduino Nano pin connected to Relay's pin const int ANALOG_THRESHOLD = 500; int analog_value; void setup() { pinMode(RELAY_PIN, OUTPUT); // set arduino pin to output mode } void loop() { analog_value = analogRead(LIGHT_SENSOR_PIN); // read the input on analog pin if(analog_value < ANALOG_THRESHOLD) digitalWrite(RELAY_PIN, HIGH); // turn on Relay else digitalWrite(RELAY_PIN, LOW); // turn off Relay }

Detailed Instructions

Plug the USB cable into both the Arduino Nano and the PC.
Launch the Arduino IDE, choose the correct board and port.
Copy the code and open it in the Arduino IDE.
Click the Upload button in the Arduino IDE to compile and upload the code to the Arduino Nano.

Emit light source towards the sensor
Check out the alteration in the relay's status

Code Explanation

Check out the line-by-line explanation contained in the comments of the source code!

Relay vs MOSFET Module

A relay is not the only way to switch a device. If your device runs on DC power, such as a 12V LED strip, a 12V solenoid lock or a 12V fan, a MOSFET module can do the same job.

	Relay	MOSFET Module
What it can do	turn the device on or off	turn the device on or off, and set its power (PWM)
Noise when switching	a click every time	no sound
How fast it switches	slow, a few milliseconds	very fast
Life	the contacts wear out after many switches	nothing moves, so it lasts long
Type of power	AC or DC	DC only
Device power supply	any voltage the relay is rated for	about 6V or more
Isolation from the Arduino	yes, between coil and contacts	yes, by the PC817 optocoupler
Good for	AC devices, or a simple ON/OFF	DC devices, or dimming and speed control

Both the relay and the MOSFET module turn on when the Arduino Nano pin is HIGH. So for a simple ON/OFF, the code above stays the same. Only the wiring changes. For AC mains devices, always use the relay.

To learn the MOSFET module pinout, wiring, PWM code and safety tips, see the Arduino Nano - MOSFET Module tutorial.

Video Tutorial

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