Arduino HC-05 Bluetooth Slave: Wiring, Code and Simulator

To build this Arduino HC-05 Bluetooth Slave project you need an Arduino Uno and a Bluetooth HC-05, a 1 kΩ resistor and a 2 kΩ resistor. Waits for the master in your other tab, answers every line with "pong" and lights the LED while STATE says it is connected. Make the 8 connections in the wiring table, upload the Arduino C++ code, and stop the master tab and watch STATE (and the LED) drop.

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 10	Bluetooth HC-05 TXD
4	Arduino Uno pin 11	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
8	Arduino Uno pin 8	Bluetooth HC-05 STATE

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 10 to Bluetooth HC-05 TXD.
Wire 4. Connect Arduino Uno pin 11 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.
Wire 8. Connect Arduino Uno pin 8 to Bluetooth HC-05 STATE.
Paste the Arduino C++ code. Paste the Arduino C++ code below into the code editor.
Run it. Press Run. Stop the master tab and watch STATE (and the LED) drop.

Arduino C++ code

// HC-05 slave: waits for the master in your other tab ("HC-05 master"),
// prints what it receives and answers every line with "pong".
// Wiring: HC-05 VCC 5V, GND, TXD -> 10, RXD <- 11 (1k/2k divider), STATE -> 8.
#include <SoftwareSerial.h>

SoftwareSerial bt(10, 11);   // RX, TX
const int STATE = 8;
String line = "";

void setup() {
  Serial.begin(9600);
  bt.begin(9600);
  pinMode(STATE, INPUT);
  pinMode(13, OUTPUT);
  Serial.println("Slave waiting to be connected...");
}

void loop() {
  digitalWrite(13, digitalRead(STATE));   // LED on while connected
  while (bt.available()) {
    char c = bt.read();
    if (c == '\n') {
      line.trim();
      Serial.print("Got: ");
      Serial.println(line);
      bt.print("pong (");
      bt.print(line);
      bt.println(")");
      line = "";
    } else {
      line += c;
    }
  }
}

What to try

Stop the master tab and watch STATE (and the LED) drop.

Frequently asked questions

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

Make 8 connections: Arduino Uno 5V to Bluetooth HC-05 VCC; Arduino Uno GND to Bluetooth HC-05 GND; Arduino Uno pin 10 to Bluetooth HC-05 TXD; Arduino Uno pin 11 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; Arduino Uno pin 8 to Bluetooth HC-05 STATE.

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?

Waits for the master in your other tab, answers every line with "pong" and lights the LED while STATE says it is connected.