HC-05 Bluetooth: Control Arduino From Your Phone (Android)

Yes — the HC-05 lets any Android phone control an Arduino Uno over Bluetooth: pair with PIN 1234, wire four jumpers plus two resistors, upload one sketch, and a free terminal app toggles the Uno’s LED. One honest warning before you buy: the HC-05 is Classic Bluetooth, so it works with Android — not iPhone.

Why does the HC-05 work with Android but not iPhone?

The HC-05 is built around a BC417 radio that speaks Classic Bluetooth 2.0+EDR with the Serial Port Profile (SPP) — it is not a BLE (Bluetooth Low Energy) module. Android supports SPP natively, so Android phones connect without fuss. Apple is a different story: iOS does not support the Serial Port Profile at all — a Bluetooth Classic accessory that streams data to an app must be MFi-certified and use Apple’s own iAP2 accessory protocol, and the HC-05 is neither, so no ordinary iPhone app can open a serial link to it. This is the number one disappointment we see with this module, so we say it up front.

If your project must talk to an iPhone, use a BLE serial module instead — or pick a board with Bluetooth built in. Our ESP32 vs ESP8266 vs Arduino Uno comparison explains why the ESP32’s built-in Bluetooth is the natural upgrade path here.

You will also want a breadboard and a few male-female jumper wires to keep the build solder-free, plus two ordinary resistors (1kΩ and 2kΩ) for the voltage divider below.

How do you wire the HC-05 to the Arduino Uno?

The HC-05 module has six male header pins: STATE, RX, TX, GND, VCC and EN. Power is easy — the board accepts 3.6–6V on VCC because an onboard regulator drops it to 3.3V, so the Uno’s 5V pin is fine. The catch is logic levels: the HC-05’s RX pin is 3.3V logic and is not 5V tolerant, while the Uno transmits at 5V. The fix is a two-resistor voltage divider: Uno D3 → 1kΩ → HC-05 RX, plus 2kΩ from HC-05 RX to GND. That divides 5V × 2000 ÷ (1000 + 2000) = 3.33V — exactly what the pin wants. In the other direction the HC-05’s 3.3V TX signal is high enough for the Uno to read directly, so no divider is needed there. Honesty note: plenty of tutorials skip the divider and the module usually survives for a while — but the pin is not rated for 5V, and the correct fix costs two resistors.

HC-05 pin	Connects to	Notes
VCC	Uno 5V	Board accepts 3.6–6V; onboard regulator makes 3.3V.
GND	Uno GND	Common ground.
TX	Uno D2	3.3V signal — the Uno reads it directly.
RX	Uno D3 via 1kΩ, plus 2kΩ to GND	Divider drops the Uno’s 5V to 3.3V.
STATE	Not connected	Optional: goes HIGH while a phone is connected.
EN	Not connected	Leave unconnected — use the onboard button for AT mode instead.

Everything here sits solder-free on the breadboard. If you later move the circuit to a permanent board, our header pin soldering guide covers the technique.

What Arduino code lets your phone control the LED?

The HC-05 wants its data on pins other than 0 and 1, so the sketch uses SoftwareSerial on pins 2 and 3, leaving the hardware serial port free for uploads and the serial monitor. Most HC-05 clone boards ship in data mode at 9600 baud. Send 1 from the phone to switch the built-in LED on pin 13 on, 0 to switch it off; the Uno echoes the new state back each time.

#include <SoftwareSerial.h>

// HC-05 TX -> Uno D2 (direct), Uno D3 -> 1k/2k divider -> HC-05 RX
SoftwareSerial bt(2, 3);  // RX, TX

const int LED_PIN = 13;   // Uno built-in LED

void setup() {
  pinMode(LED_PIN, OUTPUT);
  Serial.begin(9600);     // USB serial monitor
  bt.begin(9600);         // HC-05 data mode (default on most boards)
  bt.println("HC-05 ready. Send 1 for ON, 0 for OFF.");
}

void loop() {
  if (bt.available()) {
    char c = bt.read();
    if (c == '1') {
      digitalWrite(LED_PIN, HIGH);
      bt.println("LED ON");        // echo status back to the phone
      Serial.println("Phone sent 1 - LED ON");
    } else if (c == '0') {
      digitalWrite(LED_PIN, LOW);
      bt.println("LED OFF");
      Serial.println("Phone sent 0 - LED OFF");
    }
    // any other character (like newline) is ignored
  }
}

Once the LED obeys, the same digitalWrite can drive bigger things — swap pin 13 for a relay signal pin and your phone switches a lamp or a pump. Read our relay module safety guide first, especially the mains-voltage section.

How do you connect your Android phone to the HC-05?

The HC-05 connection is a two-step flow, and skipping step one is the most common reason “the app cannot find it”. First pair in Android itself: Settings → Bluetooth → scan → tap HC-05 → enter PIN 1234 (a few batches use 0000). Pairing only introduces the devices — nothing is connected yet.

Second, connect inside a serial terminal app. We recommend Serial Bluetooth Terminal by Kai Morich from Google Play — it supports Bluetooth Classic SPP, the profile the HC-05 uses. In the app open Devices → Bluetooth Classic, tap HC-05, and once it says connected, type 1 and send — the Uno’s LED lights and “LED ON” comes back in the terminal.

Prefer to watch the whole flow once before building? This walkthrough covers similar wiring, a similar sketch and the app pairing end to end:

How do you enter AT mode to rename or configure the HC-05?

The HC-05’s settings — name, PIN, baud rate — are changed in AT command mode, and this is where most people get stuck. The trick is timing: hold the small onboard button down while you apply power, then release it. The status LED blinks slowly — about once every two seconds — instead of the usual rapid flash — that means AT mode, which runs at a fixed 38400 baud when entered this way, regardless of your data-mode setting.

Upload this bridge sketch (same wiring as above), then power-cycle the HC-05 with the button held:

#include <SoftwareSerial.h>

SoftwareSerial bt(2, 3);  // RX, TX

void setup() {
  Serial.begin(9600);     // serial monitor side
  bt.begin(38400);        // AT mode is always 38400 baud
}

void loop() {
  // pass everything between the serial monitor and the HC-05
  if (bt.available()) Serial.write(bt.read());
  if (Serial.available()) bt.write(Serial.read());
}

Open the serial monitor at 9600 baud and set the line ending to Both NL & CR — the HC-05 ignores commands without the carriage return and newline. Type AT and you should get OK. Useful commands:

Command	What it does	Example
AT	Connection test — replies OK	AT
AT+NAME=	Renames the module (shows on the next phone scan)	AT+NAME=MyRobot
AT+PSWD=	Changes the pairing PIN	AT+PSWD=4321
AT+UART=	Sets data-mode baud, stop bits, parity	AT+UART=9600,0,0

Two quirks: on many boards, query commands such as AT+NAME? only answer while the button is held down, and clone firmware differs slightly between versions — if a valid-looking command returns ERROR, hold the button while sending it. Done? Power-cycle without the button and the module returns to data mode.

Common mistakes we see from real customers

The same handful of problems account for nearly every HC-05 support chat we get.

1. Buying it for an iPhone project. The HC-05 is Classic Bluetooth SPP and iOS will not connect to it. Android only — for iPhone, go BLE or ESP32.
2. Talking to it at the wrong baud. Data mode is 9600 on most boards; AT mode is always 38400. Garbage characters or silence almost always means these two got swapped.
3. Wiring TX to TX and RX to RX. Serial lines cross: the module’s TX goes to the Uno pin the sketch reads (D2), and its RX comes from the pin the sketch writes (D3, through the divider).
4. Expecting pairing to be the connection. Pairing in Android settings is only step one — nothing works until a terminal app opens the connection to the paired HC-05.
5. Sending AT commands without NL & CR. With the serial monitor’s line ending set to “No line ending”, the HC-05 ignores everything and beginners assume the module is dead.

FAQ

Can I use the HC-05 with an iPhone?

No. The HC-05 speaks Classic Bluetooth SPP, and iOS has no app-level support for Classic Bluetooth SPP; Bluetooth Classic data accessories need Apple MFi certification, which the HC-05 does not have. For iPhone projects use a BLE serial module, or a board with built-in Bluetooth such as the ESP32.

Why does my phone pair with the HC-05 but the app cannot connect?

Pairing and connecting are separate steps. After pairing in Android settings, you must open the connection inside a serial terminal app. Also check that no other app is already holding the connection — the HC-05 accepts only one at a time.

Why do I get garbage characters or no OK in AT mode?

Almost always baud rate or line ending. AT mode is fixed at 38400 baud no matter what your data-mode baud is, and the serial monitor must send Both NL & CR. Fix those two settings and AT should answer OK.

What is the difference between the HC-05 and HC-06?

The HC-05 can act as master or slave and has the fuller AT command set; the HC-06 is slave-only with fewer commands. For a phone-controls-Arduino project like this one, both behave the same — the phone is the master either way.

What range can I expect from the HC-05?

It is a Class 2 Bluetooth device, nominally rated around 10 metres in the open. Walls and interference reduce that in practice, so treat it as same-room range, not whole-house range.

Last updated August 2026. Stuck? Chat with us on WhatsApp.