Under the hood, the app uses the Nordic UART Service (NUS) — a simple, widely adopted GATT profile that emulates a serial link over BLE. It exposes two characteristics: a TX characteristic (UUID 6E400003) that the peripheral uses to send data to the phone, and an RX characteristic (UUID 6E400002) that the phone uses to write back to the device. The app subscribes to notifications on the TX characteristic; every time the Arduino sketch sends a temperature string, the module transmits it as a BLE notification and the app receives it immediately.

This approach is far simpler to implement than a custom GATT profile. With NUS, the firmware side is just a serial print() call — the same code you would use to debug over USB. There is no need to define custom service UUIDs, manage attribute permissions, or handle GATT read/write procedures. For sensor projects where data is small and latency of a second or two is acceptable, Nordic UART is the right choice.

For a temperature sensor transmitting one reading per second, BLE is almost always the better choice. The AT-09 draws roughly 8–15 mA during an active BLE connection — compared with 30–40 mA for an HC-05 in Classic Bluetooth mode. That difference is significant when running from a small LiPo or AA pack, and it becomes critical if you are harvesting energy or targeting months of unattended operation. More importantly, the iOS restriction on Classic Bluetooth SPP is a hard wall — there is simply no supported path for a third-party iOS app to connect to an HC-05. BLE removes that constraint entirely.

One important constraint: the app does not support BLE modules that use a non-UART GATT profile. The standard Bluetooth Health Thermometer Profile (UUID 0x1809), for example, uses a structured binary characteristic with IEEE-11073 float encoding — that is a different protocol entirely. If your module uses a custom GATT service with non-NUS UUIDs, or if it transmits data as binary rather than a decimal ASCII string, you will need a different app or a custom build.

• 1
  Build the hardware — follow the BLE Thermometer guide linked below to assemble your Arduino Uno + AT-09 circuit, connect the DS18B20 sensor, and upload the companion sketch. The sketch sends a decimal temperature string over serial every second.
• 2
  Download the app — free, no ads, available on both platforms:

  
• 3
  Enable Bluetooth on your phone — no manual pairing required for BLE. Unlike Classic Bluetooth, BLE connections are managed entirely inside the app; nothing appears in your phone’s Bluetooth settings.
• 4
  Open the app — tap the scan button. Your AT-09 or HM-10 will appear in the nearby device list within a few seconds. The module must be powered and the Arduino sketch must be running.
• 5
  Select your device — tap it to connect. The app establishes the BLE UART service connection and immediately begins receiving temperature readings. The graphical thermometer gauge will start updating within one to two seconds.
• 6
  Switch views — toggle between the Thermometer view and the Graph view using the tab bar at the bottom of the screen. The Graph view shows the last 30 readings as a line chart, useful for spotting trends or confirming sensor stability.