These two practical Arduino projects show you how to control a relay from a smartphone using an Arduino Uno either over Bluetooth or over Wi-Fi via an ESP8266-based ESP-01 module. Whether you are sending simple commands through a Bluetooth terminal app or using a UDP app over your local network, both approaches offer a clean, affordable setup with no fancy interfaces, just reliable serial communication and a solid bit of code. These projects are great building blocks for smart, wireless control, whether automating something small at home or simply experimenting.

To develop projects, we use components from the Elektor Universal Sensor Maker Kit together with the Arduino Uno development board. Unless specified otherwise, the projects developed are compatible with the Arduino Uno R3, the Arduino Uno R4 Minima, and the Arduino Uno R4 Wi-Fi.

 

Figure 1: Project block diagram.

Controlling the Arduino Uno Ports via Bluetooth Module

In this project, a Bluetooth module and a Relay module are connected to the Arduino Uno. The relay is controlled remotely from a smartphone via Bluetooth. The commands used are:

 

R=1# Activate the relay

R=0# Deactivate the relay

 

Figure 1 shows the block diagram of the project. The wiring diagram in Figure 2 illustrates how the Bluetooth module is connected to ports 3 and 4 of the Arduino Uno.

Figure 2: Project wiring diagram.
Although not depicted in the diagram, the relay module is connected to port 5. A full circuit diagram is provided in Figure 3.
Figure 3: Project circuit diagram.

Listing 1 shows the program listing (Arduino_Bluetooth.c). The SoftwareSerial library is used in this program. The Bluetooth module’s TX and RX pins are connected to ports 3 and 4 of the Arduino Uno, respectively. The relay module is connected to port 5, configured as an output, and is deactivated at the start of the program. Inside the program loop, the program waits for data to be received from the Bluetooth module. The character # is used as the data terminator. If the received data is R=1, the relay is activated. If the received data is R=0, the relay is deactivated. Note that the terminator character is not part of the received data.

Listing 1: Controlling the Arduino using the Bluetooth module.

#include <SoftwareSerial.h>
 const int BluetoothTXPin = 3; // TX pin
 const int BluetoothRXPin = 4; // RX pin
 const int Relay = 5; // Relay pin
 String cmd;
 SoftwareSerial BluetoothSerial(BluetoothTXPin, BluetoothRXPin);
 

void setup()
{
 pinMode(Relay, OUTPUT); // Relay is output
 digitalWrite(Relay, LOW); // Relay deactive
 BluetoothSerial.begin(9600); // Bluetooth
} 
 

void loop() 
{
 if(BluetoothSerial.available() > 0) // Data arrived?
 {
  cmd = BluetoothSerial.readStringUntil('#'); // Read the data
  if(cmd == "R=1") // R=1?
  {
    digitalWrite(Relay, HIGH); // Relay ON
    BluetoothSerial.println("Relay turned ON");
  }
  if(cmd == "R=0") // R=0?
  {
    digitalWrite(Relay, LOW); // Relay OFF
    BluetoothSerial.println("Relay turned OFF");
  }
 }
}

The project can be tested using a Bluetooth app on your smartphone. The author used the Bluetooth Terminal HC-05 app from mightyIT (Figure 4) on his Android smartphone.

 

Figure 4: Bluetooth app.
The steps to test the project are:

 

  • Construct the hardware and upload the program to your Arduino Uno.
  • Turn on the Bluetooth and open the Bluetooth app on your smartphone.
  • Click to SCAN for devices.
  • The Arduino Uno Bluetooth module should appear as JDY-31-SPP. Click to select it.
  • You may be asked to enter a pairing password. Enter 1234.
  • Enter the command R=1#. You should hear the relay turn on, and the message Relay turned ON should be displayed on your smartphone.
  • Enter the command R=0#. You should hear the relay turn off, and the message Relay turned OFF should be displayed on your smartphone.

 

Figure 5 shows the smartphone sending commands to the Arduino Uno.

Figure 5: Activating and deactivating the relay.

Controlling the Arduino Uno Ports via Wi-Fi

In this project, an ESP8266 module is used to add Wi-Fi capability to the Arduino Uno board (assuming you are not using the Arduino Uno R4 Wi-Fi board). The project allows a relay connected to the Arduino Uno to be activated remotely over a Wi-Fi link using a smartphone. The valid commands are:

 

R=1# Relay on

R=0# Relay off

 

Figure 6 shows the project block diagram.

Figure 6: Project block diagram.
The standard Arduino Uno does not have built-in Wi-Fi. Therefore, it must be paired with an external Wi-Fi module. One of the easiest and affordable ways to add Wi-Fi capability is by using an ESP-01 processor board. This tiny board (see Figure 7), measuring only 2.7 × 1.2 cm, is based on the ESP8266 processor chip and costs around $3.
 
Figure 7: ESP-01 processor board
The ESP-01 has the following features:

 

  • Operating voltage: +3.3 V
  • Interface: using simple AT commands over serial port/UART
  • Integrated TCP/IP protocol stack. 802.11 b/g/n
  • No external components required

 

The ESP-01 communicates with the host processor through its TX and RX serial port pins. It is an 8-pin board, with the following pin configuration:
 

VCC: +3.3 V power supply

GND: Ground

GPIO0 (or IO1): I/O pin, must be connected to +3.3 V for normal operation

GPIO2 (or IO2): General purpose I/O pin

RST: Reset pin, must be connected to +3.3 V for normal operation

CH_PD (or EN): Enable pin, must be connected to +3.3 V for normal operation

TX: Serial output pin

RX: Serial input pin


ESP-01 pins are not standard breadboard-compatible, and an adapter is required if the board is to be attached to a breadboard. Some adapter boards also include a +5 V to +3.3 V converter chip so that +5 V can be directly connected to the device. The kit supplied with the book includes such an adapter, and you should plug your ESP-01 board into the adapter board.

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The ESP-01 processor is operated with AT commands. Some commonly used AT commands for Wi-Fi connectivity include:

 

AT+RST - Reset ESP-01

AT+CWMODE - Set ESP-01 mode (in this case, Station mode)

AT+CWJAP - Set the Wi-Fi SSID name and password

AT+CPIMUX - Set connection mode (in this project, it is set to multiple connections)

AT+CIFSR - Return the IP address

AT+CIPSTART - Set TCP or UDP connection mode, destination IP address, and port number. UDP is used in this project with port 5000. The destination IP address is set to 0.0.0.0, allowing any device to send data as long as port 5000 is used. You can change this to the IP address of your smartphone to restrict access to it only.

 

Figure 8: Project wiring diagram.
Figure 8 shows the project wiring diagram where the ESP-01 TX and RX pins are connected to ports 2 and 3 of the Arduino Uno, respectively. Additionally, the S pin of the Relay board is connected to port 5.

Listing 2 shows the program listing (Arduino_wifi.c). At the beginning of the program, the TX and RX pins of the ESP-01 are assigned to software serial ports 2 and 3 of the Arduino Uno, and the relay is assigned to port 5 and is configured as an output. The function ConnectToWiFi() uses the AT commands to connect to the local Wi-Fi router. An important point to note is that the SSID and password of the router must be specified with the CWJAP command. The function then establishes a UDP communication protocol on port 5000.
Listing 2: Controlling the Arduino using the ESP8266 as Wi-Fi module.

#include <SoftwareSerial.h>
 const int ESPTxPin = 2; // TX pin
 const int ESPRxPin = 3; // RX pin
 const int Relay = 5; // Relay port
 String buf;
 SoftwareSerial ESPSerial(ESPTxPin, ESPRxPin);

// This function connects to the local WiFi router using AT commands
void ConnectToWiFi()
{
 ESPSerial.println("AT+RST");
 delay(5000);
 
 ESPSerial.println("AT+CWMODE=1");
 delay(3000);
 ESPSerial.println("AT+CWJAP=\"BTHub5-6SPN\",\"af4b5e6243\"");
 delay(5000);
 
 ESPSerial.println("AT+CIPMUX=0");
 delay(3000);
 ESPSerial.println("AT+CIPSTART=\"UDP\",\"0.0.0.0\",5000,5000,2");
 delay(5000);
 ESPSerial.println("AT+CIFSR");
 delay(5000);
}
void setup()
{
 pinMode(Relay, OUTPUT); // Relay is output
 digitalWrite(Relay, LOW); // Relay deactive
 ESPSerial.begin(115200); // ESP serial
 ConnectToWiFi(); // Connect to WiFi
} 
void loop() 
{
 if(ESPSerial.available()) // Data available?
 {
  buf = ESPSerial.readStringUntil('#'); // Read data
  int i = buf.indexOf('R'); // Look for 'R'
  if(i > 0) // i is index of R
  {
   if(buf[i] == 'R' && buf[i+1] == '=' && buf[i+2] == '1')
   {
    digitalWrite(Relay, HIGH);
   }
   if(buf[i] == 'R' && buf[i+1] == '=' && buf[i+2] == '0')
   {
    digitalWrite(Relay, LOW);
   }
  }
 }
} 

Inside the main program loop, the function ConnectToWiFi() is called to connect to the local Wi-Fi router. Then, the program waits to receive commands from the smartphone over the UDP port. Character R is looked for in the received data using the built-in function indexOf(). This function returns -1 if the required character is not found; otherwise, it returns the index where the required character is found. If the received command is R=1, the relay is turned on. If the received command is R=0, the relay is turned off.

Before starting the test, you should install a UDP app on your Android smartphone. There are many freely available UDP apps in the Play Store. The app used in this project is called UDP/TCP Widget by K.J.M, as shown in Figure 9.

 

Figure 9: UDP/TCP Widget apps for Android.
The steps to test the program are as follows:

 

  • Construct the circuit.
  • Download the program to your Arduino.
  • Start the UDP/TCP Widget app on your smartphone.
  • Click the gear symbol and set the protocol to UDP. Enter the IP address of your ESP-01 (e.g., 192.168.1.160 for the author’s ESP-01), and set the port to 5000 as shown in Figure 10.
  • Click the MESSAGE menu item, select Text (UTF-8) as the format, and enter command R=1# to turn on the relay or R=0# to turn it off. Select LF\n as the Terminator.
  • Click the red right arrow and then the SEND button to send the command to the Arduino Uno.

 

You can find the IP address of the ESP-01 by scanning all the devices on the local Wi-Fi router. For example, the Android app Who Uses My WiFi – Net Scanner by Phuongpn can show the IP addresses of all devices connected to your router.

 

Figure 10: Configure the apps.
These projects demonstrate how easily the Arduino Uno can be enhanced with wireless control, whether through classic Bluetooth or by adding Wi-Fi using the ESP-01. More than just practical exercises, these projects serve as a foundation for exploring more advanced wireless communication techniques and IoT applications.

Editor’s Note: This article (250325-01) appers in Elektor September/October 2025. It is an excerpt from the Elektor Universal Sensor Maker Kit. It was formatted and lightly edited to match Elektor Magazine’s conventions and page layout.

Questions About These Arduino Projects?

If you have questions or comments about these Arduino projects, email the author at d.ibrahim@btinternet.com or Elektor at editor@elektor.com.

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