Smart Extension Board Using Arduino and a Custom Android App

Yes, an Arduino Uno, HC-05 Bluetooth module, four-channel relay board, and Android app can control four outlets independently—but the design should be treated as a low-voltage educational prototype, not a certified household power strip. Build and test the controller with LEDs or low-voltage loads first. Any connection to household mains requires correctly rated components, enclosure design, fusing, separation, strain relief, testing, and appropriate electrical expertise.

What the project does

This is a local Bluetooth relay controller rather than a cloud-connected smart-home product. The phone sends a command over Bluetooth Classic, the HC-05 presents that command to the Arduino over UART, and the Arduino changes one of four relay outputs. The relay contacts then switch the corresponding outlet.

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Android app
    ↓ Bluetooth Classic serial
HC-05 module
    ↓ UART
Arduino Uno
    ↓ GPIO
4-channel relay module
    ↓ switching contacts
Four outlets

The reference implementation, published by Hackster on March 11, 2023, uses an Arduino Uno, HC-05, four relays, Arduino IDE, and MIT App Inventor. It supports separate controls for four outlets and commands for switching everything on or off.

Safety boundary: prototype first, mains later

Do not begin by wiring household AC to a breadboard or exposed relay module. Mains voltage can cause fatal shock, fire, arc faults, and damage to connected equipment. A generic relay board, jumper wires, optocouplers, or plastic project box does not automatically make a power strip safe or code-compliant.

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• Fault-Tolerant Design: Fault Tolerant Design, Even if the Control Line is Broken, the Relay will not Operate;All Interfaces of Relay can be Wired Out Through the Terminals Directly,Normally Open and Normally Closed
• Optocoupler Isolation:1 Channel Relay Board use Optocoupler Isolation that has Strong Driving Ability and Stable Performance ,The Isolation Circuit Prevent Damages to I / O Port by Relay Switch Current
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For the first build, replace the outlets with LEDs, low-voltage lamps, or another isolated test load. Keep the Arduino, Bluetooth module, and relay inputs on the low-voltage side. Treat mains integration as a separate electrical-design task that may require a qualified electrician or engineering review.

A mains version would need, at minimum:

• Relays rated for the local voltage, current, load type, and inrush current.
• Mains-rated wire, terminals, fuse or circuit protection, and an enclosed certified power supply.
• Physical separation, suitable creepage and clearance, insulation barriers, and no accessible live conductors.
• Strain relief or cable glands where cords enter the enclosure.
• Correct switching of the intended conductor under local electrical rules.
• Verification of grounding, polarity, insulation, enclosure integrity, and startup behavior.

Do not describe the finished project as UL-listed, ETL-listed, certified, or code-compliant unless it has actually been evaluated to the applicable standard.

Parts list

Reference prototype

• Arduino Uno
• HC-05 Bluetooth Classic module
• Four-channel relay module
• USB cable and computer
• Jumper wires
• Suitable low-voltage power supply
• LEDs or low-voltage loads for testing

The Arduino Uno Rev3 specification lists a 5 V operating voltage, 14 digital I/O pins, six analog inputs, a 16 MHz clock, 32 KB flash, and 2 KB SRAM. Four GPIO pins are sufficient for relay inputs, but an Arduino pin should not directly power a relay coil. The relay board’s driver transistor, flyback protection, power rail, and total coil current must be considered separately.

Recommended additions

• Reputable relay module with published contact and isolation ratings
• Certified enclosed AC-to-DC supply, if a mains prototype is ever developed
• Mains-rated terminal blocks, fuse holder, wire, heat-shrink, and strain relief
• Insulating barriers and a properly designed enclosure
• LED indicators and manual override buttons
• Defined power-on behavior that leaves every outlet off
• Hardware emergency-off control for any unattended installation

Suggested low-voltage pin map

Before connecting anything, confirm the relay board’s input voltage, VCC requirements, ground arrangement, whether it is genuinely opto-isolated, and whether its inputs are active-high or active-low. Do not infer safety ratings from the presence of an optocoupler.

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HC-05 Bluetooth architecture

The HC-05 normally provides a Bluetooth Classic serial link. Pairing is the operating system’s relationship with the module; connection is the active data link; the serial protocol is the characters exchanged after connection. The phone does not control the relay directly—it writes commands to the HC-05.

Bluetooth Classic is not BLE. An HC-05 serial application cannot simply be copied to an Arduino board using BLE. A BLE implementation needs a GATT service, a writable characteristic, Android scanning and GATT connection logic, and defined service and characteristic UUIDs.

The Arduino UNO R4 WiFi includes Wi-Fi and Bluetooth 5 LE through an ESP32-S3 module, but it is not a drop-in replacement for HC-05 firmware. Its wireless module uses a 3.3 V logic domain while the main Arduino MCU operates at 5 V; incompatible signals must not be connected directly.

Serial wiring and the Uno’s hardware port

The reference code identifies HC-05 communication but includes a commented SoftwareSerial declaration while reading from Serial at 9600 baud. On an Uno, Serial uses the same hardware serial pins connected to USB. That can complicate uploading firmware, using the Serial Monitor, and Bluetooth communication at the same time.

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A clearer prototype uses a separate software serial port. Cross TX and RX, connect the required ground, and verify the voltage requirements of the particular HC-05 breakout board.

#include <SoftwareSerial.h>

SoftwareSerial btSerial(6, 7);  // Arduino RX, TX

const byte RELAY_1 = 2;
const byte RELAY_2 = 3;
const byte RELAY_3 = 4;
const byte RELAY_4 = 5;

// Change these after testing the module with mains disconnected.
const bool RELAY_ON_LEVEL = HIGH;

void setRelay(byte pin, bool on) {
  digitalWrite(pin, on ? RELAY_ON_LEVEL : !RELAY_ON_LEVEL);
}

void setup() {
  pinMode(RELAY_1, OUTPUT);
  pinMode(RELAY_2, OUTPUT);
  pinMode(RELAY_3, OUTPUT);
  pinMode(RELAY_4, OUTPUT);

  setRelay(RELAY_1, false);
  setRelay(RELAY_2, false);
  setRelay(RELAY_3, false);
  setRelay(RELAY_4, false);

  btSerial.begin(9600);
}

void allRelays(bool on) {
  setRelay(RELAY_1, on);
  setRelay(RELAY_2, on);
  setRelay(RELAY_3, on);
  setRelay(RELAY_4, on);
}

void loop() {
  if (!btSerial.available()) return;

  char command = btSerial.read();

  switch (command) {
    case 'A': setRelay(RELAY_1, true);  break;
    case 'B': setRelay(RELAY_1, false); break;
    case 'C': setRelay(RELAY_2, true);  break;
    case 'D': setRelay(RELAY_2, false); break;
    case 'E': setRelay(RELAY_3, true);  break;
    case 'F': setRelay(RELAY_3, false); break;
    case 'G': setRelay(RELAY_4, true);  break;
    case 'H': setRelay(RELAY_4, false); break;
    case 'I': allRelays(true);  break;
    case 'J': allRelays(false); break;
  }
}

This is an illustrative low-voltage example, not a verified mains-control design. “Relay on” must be defined by your module’s polarity and by whether the load is wired through normally open or normally closed contacts.

Command mapping

Single-character commands are convenient for a demonstration but fragile as a long-term protocol. A more extensible format would use framed, human-readable messages:

R1:ON

R1:OFF

ALL:OFF

STATE?

The controller could answer with OK R1 ON, ERR UNKNOWN_COMMAND, or STATE R1=0,R2=1,R3=0,R4=0. Acknowledgements matter because a successful Bluetooth write does not prove that the relay changed or that the outlet is electrically safe.

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Relay polarity and startup testing

Relay modules may be active-high or active-low. Some inputs briefly float during reset, causing an unwanted click. The physical outlet state also depends on whether the circuit uses normally open or normally closed contacts.

Test the controller with mains disconnected:

1. Power the Arduino and relay board from an appropriate low-voltage source.
2. Observe the relay indicators and listen for unintended activation during reset.
3. Use a multimeter to verify contact behavior.
4. Confirm which logic level means energized.
5. Confirm that startup leaves every relay in the intended off state.
6. Repeat the reset and power-cycle test several times.

Prefer a defined all-off state after boot. Do not automatically restore an old on-state unless that behavior has been deliberately designed and tested.

Building the Android controller

The reference project uses MIT App Inventor, which is suitable for a basic educational interface. The app should provide:

• A paired-device picker
• Connect and disconnect status
• Four outlet controls
• All-on and all-off controls
• Disabled controls until a valid connection exists
• Clear connection and command-error states
• State indicators based on controller acknowledgements rather than button presses alone

Android 12 and later permissions

Older Bluetooth tutorials often omit the changes introduced for apps targeting Android 12/API 31 or later. According to Android’s Bluetooth permission documentation, scanning requires BLUETOOTH_SCAN, communicating with paired devices requires BLUETOOTH_CONNECT, and advertising requires BLUETOOTH_ADVERTISE. The latter is not needed when the phone only connects to an HC-05.

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These are runtime permissions. Request only those the app actually needs, handle denial, check that Bluetooth hardware exists, and handle the case where Bluetooth is disabled. A manifest pattern for a basic scanner-and-connector is:

<uses-permission
    android:name="android.permission.BLUETOOTH"
    android:maxSdkVersion="30" />

<uses-permission
    android:name="android.permission.BLUETOOTH_ADMIN"
    android:maxSdkVersion="30" />

<uses-permission
    android:name="android.permission.BLUETOOTH_SCAN"
    android:usesPermissionFlags="neverForLocation" />

<uses-permission
    android:name="android.permission.BLUETOOTH_CONNECT" />

<uses-permission
    android:name="android.permission.ACCESS_FINE_LOCATION"
    android:maxSdkVersion="30" />

The location declaration depends on the app’s actual behavior and Android version. Follow the current Bluetooth setup guidance rather than requesting every permission by default.

Connection flow

1. Check whether Bluetooth hardware is available.
2. Prompt the user to enable Bluetooth if necessary.
3. Request the relevant runtime permissions.
4. Find or select the paired HC-05.
5. Connect using Bluetooth Classic serial.
6. Enable outlet controls only after connection succeeds.
7. Handle disconnection and show the state as unknown until it is confirmed again.

Do not use an HC-05 app’s Classic Bluetooth connection code for an UNO R4 WiFi or ESP32 BLE service without rewriting the communication layer.

Power-supply design

Use USB or a suitable regulated supply during low-voltage development. Do not build an improvised capacitive or transformerless mains supply. The Uno documentation supports USB at 5 V and recommends 7–12 V through the DC jack or VIN; directly applying power to the 5 V or 3.3 V pins bypasses regulation and can damage the board.

Account for the Arduino, HC-05, relay-board logic, and all four relay coils. Verify startup and transient current, provide useful margin, and check for voltage droop when several relays energize simultaneously. A small USB port or linear regulator should not be assumed to power four relay coils safely.

Failure-mode checklist

Testing matrix

Which platform should you choose?

For a classroom or maker demonstration, Uno plus HC-05 remains understandable and inexpensive to prototype. For a new connected product, an ESP32-class board or UNO R4 WiFi is generally more capable. For routine household operation, a certified commercial power strip or smart plug is the safer choice.

Final recommendation

Build the four-channel controller as a low-voltage prototype, verify relay polarity and startup behavior, and make the Android app report acknowledged controller state. Do not treat the original tutorial’s relay board and enclosure as proof of safe mains construction. Only proceed to household AC after the switching components, supply, protection, enclosure, insulation, wiring, and local requirements have been reviewed by someone qualified to do that work.

Quick Recap

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AEDIKO 4pcs DC 5V Relay Module - 1 Channel Relay Switch Board with Optocoupler Isolation, High or Low Level Trigger

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Bestseller No. 2

ELEGOO 37-in-1 Sensor Modules Kit with Tutorial Compatible with Arduino

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