SENSORS/ACTUATORS

INTERNET OF THING (IoT)

ESP32 S3 UNO - Light Sensor Control Relay

This ESP32 S3 Uno light sensor relay tutorial shows you how to switch a relay on and off from the light level around the ESP32 S3 Uno form board. You can use a raw LDR photoresistor or an LDR light sensor module. When it gets dark, the relay turns on, so a light bulb, LED strip, motor or solenoid lock can start working by itself.

In this tutorial, you will:

Wire an LDR light sensor and a relay module to the ESP32 S3 Uno
Read the light level with analogRead() and compare it with a threshold
Use a digital LDR module output to control a relay
Upload the Arduino code from Arduino IDE and test it

Hardware Preparation

1	×	ESP32 S3 Uno-form Board
1	×	USB Cable Type-A to Type-C (for USB-A PC)
1	×	USB Cable Type-C to Type-C (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

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 .

A bare LDR light sensor is very cheap, but you need an extra resistor to wire it. If you want a simpler circuit, pick an LDR light sensor module instead. It has the resistor on board and gives you a digital output.

Overview of Relay and Light Sensor

This project joins two parts you may already know. If one of them is new to you, read its own tutorial first. You will learn the pinout, how it works and how to code it.

In short, the light sensor tells the board how bright it is. The relay is an electric switch that the board can turn on and off. The ESP32 S3 Uno sits in the middle and decides when the relay should switch.

ESP32 S3 Uno Pinout

The image below shows the pinout diagram of the ESP32 S3 Uno form board. Use it to find the A0, D9 and D12 headers and their GPIO numbers when you wire the circuit.

Wiring Diagram

There are two ways to build this project. Choose the diagram that matches your light sensor. In both cases the relay input goes to D9 (GPIO46).

Option 1 - Raw LDR light sensor (analog)

The photoresistor and the 10 kΩ resistor form a voltage divider. Their middle point goes to A0.

The wiring diagram between ESP32 S3 Uno Light Sensor Relay

This image is created using Fritzing. Click to enlarge image

Component Pin	ESP32 S3 Uno Pin
Light sensor pin 1	3.3V
Light sensor pin 2	A0 (GPIO2)
10 kΩ resistor, end 1	A0 (GPIO2)
10 kΩ resistor, end 2	GND
Relay VCC	5V
Relay GND	GND
Relay IN	D9 (GPIO46)

Option 2 - LDR light sensor module (digital)

The module has its own resistor and comparator. Its DO pin goes to D12.

The wiring diagram between ESP32 S3 Uno LDR Light Sensor module relay

This image is created using Fritzing. Click to enlarge image

Component Pin	ESP32 S3 Uno Pin
LDR module VCC	3.3V
LDR module GND	GND
LDR module DO	D12 (GPIO13)
Relay VCC	5V
Relay GND	GND
Relay IN	D9 (GPIO46)

D9 is GPIO46 on the ESP32 S3 Uno. It is a boot strapping pin. The board reads it at startup to pick the boot mode. Some relay modules pull their IN pin up or down, and this can stop the board from booting or uploading code. If that happens, unplug the relay IN wire, upload the code, and then plug it back in.

Power the light sensor or the LDR module from 3.3V, not 5V. The ESP32 S3 Uno pins are NOT 5V tolerant. A 5V signal on A0 (GPIO2) or D12 (GPIO13) can damage the board. The relay module may use 5V on its VCC pin, because its IN pin only receives a signal from the board.

If you use the optional solenoid lock, connect it to the relay's COM and NO screw terminals, together with the 12V power adapter and the DC power jack. Never power the lock from the ESP32 S3 Uno.

ESP32 S3 Uno Code

There are two sketches, one for each wiring option. Both of them turn the relay on when it is dark and turn it off when there is light.

Code for the raw LDR light sensor

This sketch reads A0 (GPIO2) again and again. When the value is lower than the threshold (2000), it is dark, so the relay turns on. Otherwise, the relay turns off.

/* * This ESP32 S3 Uno code was developed by newbiely.com * * This ESP32 S3 Uno code is made available for public use without any restriction * * For comprehensive instructions and wiring diagrams, please visit: * https://newbiely.com/tutorials/esp32-s3-uno/esp32-s3-uno-light-sensor-controls-relay */ #define LIGHT_SENSOR_PIN 2 // ESP32 S3 Uno pin A0 (GPIO2) connected to light sensor's pin #define RELAY_PIN 46 // ESP32 S3 Uno pin D9 (GPIO46) connected to Relay's pin #define ANALOG_THRESHOLD 2000 // 12-bit ADC (0-4095) // variables will change: int analogValue; void setup() { // set the ADC attenuation to 11 dB (up to ~3.3V input) analogSetAttenuation(ADC_11db); pinMode(RELAY_PIN, OUTPUT); // set ESP32 S3 Uno 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 }

In setup(), the code calls analogSetAttenuation(ADC_11db). By default, the ESP32 S3 Uno ADC can only measure a small voltage. With 11 dB attenuation, the analog pin can read the full range, from 0V up to about 3.3V. Without this line, the readings reach 4095 too early and the light values look wrong.

The threshold of 2000 is the old 0 to 1023 value of 500 scaled up for the 12-bit ADC (0 to 4095). Change it to fit the light in your room.

Code for the LDR light sensor module

This sketch reads the digital DO pin on D12 (GPIO13). The module outputs HIGH when there is no light, so the relay turns on. It also prints the state to the Serial Monitor.

/* * This ESP32 S3 Uno code was developed by newbiely.com * * This ESP32 S3 Uno code is made available for public use without any restriction * * For comprehensive instructions and wiring diagrams, please visit: * https://newbiely.com/tutorials/esp32-s3-uno/esp32-s3-uno-light-sensor-controls-relay */ #define LIGHT_SENSOR_PIN 13 // ESP32 S3 Uno pin D12 (GPIO13) connected to light sensor's pin #define RELAY_PIN 46 // ESP32 S3 Uno pin D9 (GPIO46) connected to Relay's pin // variables will change: int analogValue; void setup() { Serial.begin(9600); // initialize serial communication pinMode(LIGHT_SENSOR_PIN, INPUT); // initialize the ESP32 S3 Uno's pin as an input pinMode(RELAY_PIN, OUTPUT); // initialize the ESP32 S3 Uno's pin as an output mode } void loop() { int light_state = digitalRead(LIGHT_SENSOR_PIN); if (light_state == HIGH) { Serial.println("The light is NOT present"); digitalWrite(RELAY_PIN, HIGH); // turn on Relay } else { Serial.println("The light is present"); digitalWrite(RELAY_PIN, LOW); // turn off Relay } }

Detailed Instructions

Follow these steps in order:

New to the ESP32 S3 Uno? Follow ESP32 S3 Uno - Getting Started first.
Wire it up as shown in the diagram that matches your sensor.
Connect the board to your computer with a USB Type-C cable.
Open Arduino IDE, choose the ESP32S3 Dev Module board and the correct COM port.
Copy the code for your sensor and paste it into Arduino IDE.
Upload the code by clicking the Upload button.
Shine a light on the sensor, then cover it with your hand.
Watch the relay. It clicks on in the dark and clicks off in the light.
Open the Serial Monitor at 9600 baud if you use the LDR module sketch.

Newbiely | Arduino IDE 2.3.8

──

☐

Newbiely.ino

···

8 Serial.println("Hello World!");

Output

Serial Monitor

Message (Enter to send message to 'ESP32S3 Dev Module' on 'COM15')

New Line

9600 baud

The light is present The light is present The light is NOT present The light is NOT present The light is present

Ln 11, Col 1

ESP32S3 Dev Module on COM15

Tip: On the LDR module, turn the small blue potentiometer to set how dark it must be before the relay switches.

Code Explanation

Each line of the two sketches has a comment that says what it does. Read the comments in the code above to follow the logic step by step.

Video Tutorial

Watch the video below to see this ESP32 S3 Uno project step by step.

FAQ

Can I power the relay module from the 3.3V pin of the ESP32 S3 Uno?

Most relay modules have a 5V coil, so power their VCC from the 5V pin. The IN pin can still take the 3.3V signal from the ESP32 S3 Uno on most modules. If your relay does not click, use a 3V relay module or a module with a transistor driver.

Why is the threshold 2000 and not 500 like on the UNO R4?

The ESP32 S3 Uno has a 12-bit ADC, so analogRead() returns 0 to 4095. The UNO R4 sketch used 0 to 1023. The value 500 times 4 is about 2000, so the switch point stays about the same.

Why does the code call analogSetAttenuation(ADC_11db)?

It sets the ESP32 S3 Uno ADC input range to about 0 to 3.3V, so the full light sensor signal maps to 0 to 4095. Without it the range is smaller and readings saturate early. The UNO R4 does not need this line, as it is ESP32-only. Keep the sensor output at 3.3V or less.

Is it safe to use D9 for the relay?

D9 is GPIO46, a boot strapping pin. It works fine as an output after the board starts. But if the relay module holds this pin at a wrong level during startup, the board may not boot or upload. If you have problems, unplug the relay IN wire while you upload code.

What if my project needs more pins than the Uno headers give?

The ESP32 S3 Uno has two extra rows of holes with more GPIOs. GPIO15 and GPIO16 are I/O, PWM and analog. GPIO45 and GPIO35 to GPIO42 are I/O and PWM. GPIO47 and GPIO48 are output only, with PWM. Solder pin headers to use them and write the GPIO number in code. For a second light sensor, use GPIO15 or GPIO16. For more relays, GPIO35 to GPIO42 are a good choice. Be careful with two things. First, GPIO47 and GPIO48 may run at 1.8V instead of 3.3V on some modules (especially ones with "V" in the name, like R8V or R16V), so 3.3V devices can be damaged or act strangely. Second, GPIO0, GPIO3 (D6), GPIO45 and GPIO46 (D9) are boot strapping pins, and a wrong connection can stop the board from booting or uploading code. Use the other extra pins first.

Can I switch a 220V lamp with this relay?

The relay can switch mains power, but mains voltage is dangerous. Only do it if you know how to work safely with high voltage. For learning, use a 12V solenoid lock or a 12V LED strip instead.

Troubleshooting

Problem	Possible Cause	Solution
No COM port in Arduino IDE	USB driver is missing or the board is not in upload mode	Install the CP210x or CH340 driver, or hold BOOT while pressing RESET
Upload fails or the board does not start	The relay module holds D9 (GPIO46) at a wrong level at boot	Unplug the relay IN wire, upload, then plug it back in
Relay never switches	Relay VCC or GND is not connected, or the relay needs 5V	Power the relay VCC from 5V and check the GND wire
Relay is always on or always off	The threshold does not match your room light	Print the analogRead() value and set the threshold between the dark and light values
Readings stay at 4095	analogSetAttenuation(ADC_11db) is missing or the sensor gets 5V	Keep the attenuation line and power the sensor from 3.3V
Relay works the opposite way	The relay module is active LOW	Swap HIGH and LOW in the digitalWrite() lines
LDR module never changes state	The module sensitivity is set too high or too low	Turn the small potentiometer on the module while you cover the sensor
Serial Monitor shows strange characters	Wrong baud rate	Set the Serial Monitor to 9600 baud

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