Arduino HC-05 Bluetooth on Hardware Serial: Wiring, Code and Simulator

To build this Arduino HC-05 Bluetooth on Hardware Serial project you need an Arduino Uno and a Bluetooth HC-05, a 1 kΩ resistor and a 2 kΩ resistor. An HC-05 on pins 0 and 1: the phone’s text goes straight into Serial, and the sketch echoes it back in capitals. Make the 7 connections in the wiring table, upload the Arduino C++ code, and pair from the Radio tab and type a word. Pins 0/1 are also the serial monitor’s port, so it shows up there too.

Open this circuit in the free Arduino Uno simulator

Parts list

Qty	Part	What it does
1	Arduino Uno	the microcontroller board
1	Bluetooth HC-05	Bluetooth serial module: VCC 5 V, GND, TXD → the board’s RX pin, RXD ← the board’s TX pin (through a 1k/2k divider on a 5 V board). Starts at 9600 baud; hold EN high at power-up for AT commands at 38400. Pair it from the Radio tab’s phone terminal, or with a master HC-05 in another circuit (open it in a second tab).
1	1 kΩ resistor	Limits current. 220 Ω in series with an LED; 10 kΩ as a pull-up or pull-down.
1	2 kΩ resistor	Limits current. 220 Ω in series with an LED; 10 kΩ as a pull-up or pull-down.

Wiring

#	From	To
1	Arduino Uno 5V	Bluetooth HC-05 VCC
2	Arduino Uno GND	Bluetooth HC-05 GND
3	Arduino Uno pin 0	Bluetooth HC-05 TXD
4	Arduino Uno pin 1	1 kΩ resistor leg 1
5	1 kΩ resistor leg 2	Bluetooth HC-05 RXD
6	2 kΩ resistor leg 1	Bluetooth HC-05 RXD
7	Arduino Uno GND	2 kΩ resistor leg 2

Step by step

Open the simulator. Open the BitForBytes Embedded Lab in your browser and start from the Arduino Uno. No install or account is needed.
Add the parts. Add a Bluetooth HC-05, a 1 kΩ resistor and a 2 kΩ resistor from the parts palette.
Wire 1. Connect Arduino Uno 5V to Bluetooth HC-05 VCC.
Wire 2. Connect Arduino Uno GND to Bluetooth HC-05 GND.
Wire 3. Connect Arduino Uno pin 0 to Bluetooth HC-05 TXD.
Wire 4. Connect Arduino Uno pin 1 to 1 kΩ resistor leg 1.
Wire 5. Connect 1 kΩ resistor leg 2 to Bluetooth HC-05 RXD.
Wire 6. Connect 2 kΩ resistor leg 1 to Bluetooth HC-05 RXD.
Wire 7. Connect Arduino Uno GND to 2 kΩ resistor leg 2.
Paste the Arduino C++ code. Paste the Arduino C++ code below into the code editor.
Run it. Press Run. Pair from the Radio tab and type a word. Pins 0/1 are also the serial monitor’s port, so it shows up there too.

Arduino C++ code

// HC-05 on the hardware serial pins: whatever the phone sends is echoed back
// in capitals, and printed to the serial monitor too (pins 0/1 are the same
// port the monitor uses).
// Wiring: HC-05 VCC 5V, GND, TXD -> pin 0 (RX), RXD <- pin 1 (TX) through a 1k/2k divider.
void setup() {
  Serial.begin(9600);
}

void loop() {
  if (Serial.available()) {
    char c = Serial.read();
    if (c >= 'a' && c <= 'z') c = c - 'a' + 'A';
    Serial.write(c);
  }
}

What to try

Pair from the Radio tab and type a word. Pins 0/1 are also the serial monitor’s port, so it shows up there too.

Frequently asked questions

How do I wire the HC-05 Bluetooth on Hardware Serial project on an Arduino Uno?

Make 7 connections: Arduino Uno 5V to Bluetooth HC-05 VCC; Arduino Uno GND to Bluetooth HC-05 GND; Arduino Uno pin 0 to Bluetooth HC-05 TXD; Arduino Uno pin 1 to 1 kΩ resistor leg 1; 1 kΩ resistor leg 2 to Bluetooth HC-05 RXD; 2 kΩ resistor leg 1 to Bluetooth HC-05 RXD; Arduino Uno GND to 2 kΩ resistor leg 2.

What parts do I need for this Arduino project?

An Arduino Uno and a Bluetooth HC-05, a 1 kΩ resistor and a 2 kΩ resistor.

Can I simulate this Arduino project online for free?

Yes. The BitForBytes Embedded Lab runs this exact circuit and Arduino C++ code in your browser for free, with no install and no hardware. The sketch is compiled with the real Arduino toolchain and runs on an emulated ATmega328P.

What does the code do?

An HC-05 on pins 0 and 1: the phone’s text goes straight into Serial, and the sketch echoes it back in capitals.