For battery-powered deployments — a wireless temperature monitor in a greenhouse, server room, or outdoor enclosure — BLE makes far more sense than Classic Bluetooth. The AT-09 running at 10 mA versus the HC-05 at 35 mA is the difference between weeks and days on a small LiPo cell. The absence of any pairing step is also a practical advantage when handing a finished device to someone who isn’t technically minded.

• 1
  DS18B20 VCC → Arduino 5 V
  The DS18B20 operates from 3.0 V to 5.5 V — connecting to the 5 V rail is the standard and most reliable option.
• 2
  DS18B20 GND → Arduino GND
  Connect to any GND pin on the Arduino. All grounds in the circuit must share a common reference.
• 3
  DS18B20 DATA → Arduino Pin 2 (with 4.7 kΩ pullup)
  Place the 4.7 kΩ resistor between the DATA line and the 5 V rail. Without it, the 1-Wire bus will be unreliable and readings will fail intermittently.
• 4
  AT-09 VCC → Arduino 3.3 V
  Important: the AT-09 is a 3.3 V device. Connecting VCC to the 5 V rail can damage the module. Use the dedicated 3.3 V output pin on the Arduino.
• 5
  AT-09 GND → Arduino GND
  Tie to the same common ground as the DS18B20.
• 6
  AT-09 TXD → Arduino Pin 0 (RX) & AT-09 RXD → Arduino Pin 1 (TX) via voltage divider
  The AT-09 TX output is 3.3 V, which the Arduino Uno RX pin (5 V tolerant) can read safely — no divider needed on that direction. However, Arduino TX outputs 5 V, which exceeds the AT-09 RX maximum. Use a simple voltage divider: a 1 kΩ resistor from Arduino Pin 1 to the AT-09 RXD pin, and a 2 kΩ resistor from that junction to GND. This brings 5 V down to approximately 3.3 V.

Reference wiring diagram — same layout as the HC-05 project except AT-09 VCC connects to Arduino 3.3 V (not 5 V). DS18B20 data on pin 2 with 4.7 kΩ pullup, AT-09 TXD → pin 0 (RX), AT-09 RXD ← pin 1 (TX) via 1 kΩ / 2 kΩ voltage divider.

The overall layout is essentially identical to the HC-05 project — the sole structural difference is the power rail. The HC-05 takes 5 V; the AT-09 requires 3.3 V. Get that wrong and you risk permanently damaging the BLE module.

// Include the libraries
#include <OneWire.h> 
#include <DallasTemperature.h>

// Data wire goes to pin 2 on the Arduino 
#define ONE_WIRE_BUS 2 

// Setup a oneWire instance
OneWire oneWire(ONE_WIRE_BUS); 

// Pass our oneWire reference to Dallas Temperature. 
DallasTemperature sensors(&oneWire);

void setup(void) 
{ 
 // start serial port at 9600 baud — must match AT-09 default baud
 Serial.begin(9600); 
 Serial.println("Temperature Demo started"); 
 // Start up the sensor library 
 sensors.begin(); 
} 

void loop(void) 
{ 
  // request temperature reading
 sensors.requestTemperatures(); 
 // print first sensor value
 Serial.print(sensors.getTempCByIndex(0));  
 // send \r\n termination the app expects
 Serial.println("\r\n");
 // wait 1 second before next reading
 delay(1000); 
}

The OneWire library handles the low-level 1-Wire bus protocol — it manages the precise timing required to communicate with the DS18B20’s 64-bit ROM and initiate temperature conversions. DallasTemperature sits on top of it and abstracts the DS18B20-specific commands: you call requestTemperatures() to trigger a conversion and getTempCByIndex(0) to read the result in Celsius from the first sensor on the bus. The serial output — a plain decimal number followed by \r\n — is forwarded by the AT-09 directly to the connected phone via the BLE UART (Nordic NUS) notify characteristic. The phone app reads each terminated line as one temperature sample.

• 1
  Install libraries — in Arduino IDE: Sketch → Include Library → Manage Libraries → search DallasTemperature → install. The IDE will prompt to also install OneWire as a dependency; accept it.
• 2
  Upload the sketch — click Verify, then Upload. Wait for “Done uploading” in the status bar. If you get a serial port error, disconnect the AT-09 RX/TX wires before uploading — they share the hardware UART.
• 3
  Verify via serial monitor — press Ctrl+Shift+M, set baud to 9600. You should see temperature values printing every second. If you see garbage characters, the baud rate doesn’t match.
• 4
  Power the Arduino — once powered, the AT-09 LED will blink approximately once per second. This is normal — it indicates the module is advertising and waiting for a BLE connection.
• 5
  Open the BLE Thermometer app on your phone — available on iOS App Store and Google Play. No system-level Bluetooth pairing is needed; BLE handles discovery entirely within the app.
• 6
  Tap the device name to connect — the AT-09 LED will switch from blinking to a steady glow when a connection is established. Temperature readings will begin appearing in the app within 2 seconds.