Arduino and the AT-09 Bluetooth LE Module

The AT-09 is a small Bluetooth LE module that an Arduino controls with AT commands over a serial line. This project is a series of five sketches that grow from a first “AT” test into an Arduino that sends the temperature of a DHT22 over Bluetooth, so you can read it in a BLE app on your phone.

The AT-09 module

The AT-09 is one of the clones of the HM-10 module. Both are built on a Texas Instruments CC2541 chip, according to Arik Yavilevich, and they are sold under names such as CC41-A and MLT-BT05. They are not identical to the original:

• The genuine HM-10 expects commands without a line ending, and the CC41-A clones expect a carriage return and a new line. The sketches use println(), which sends both.
• The set of AT commands is smaller than that of the HM-10, and less documented, see EDN.
• AT commands only work while no phone is connected. After a connection is made, they are treated as data, see Martyn Currey.
• The serial pins of the module work on 3.3 V. On a 5 V Arduino, bring the Arduino TX down to 3.3 V with a voltage divider of 1 kΩ and 2 kΩ, as Martyn Currey advises for the HM-10.

Check the documentation of your own module, since clones differ between batches.

Wiring

The sketches use SoftwareSerial on pins 2 and 3 of the Arduino at 9600 baud, so the hardware serial port stays free for the serial monitor.

The later sketches declare pins 4 and 5 for the EN and STATE pins of the module. They are not used in the code.

The five versions

The header of the 1.0.7D file already says 1.0.7.E, so the two files are the same sketch. The name LightBlue is also the name of a popular BLE app.

What the sketches do, and what they should do

Reading the final sketch line by line:

• Setup. At every start it sends AT, AT+ROLE0, AT+UUID0xFFE0, AT+CHAR0xFFE1 and AT+NAMELightBlue and prints each reply. FFE0 and FFE1 are the service and characteristic that the HM-10 uses by default for its serial data, which is what apps such as LightBlue expect. The module normally keeps these settings, so a single run is enough, but repeating them does no harm.
• Loop. Every two seconds it reads the DHT22, skips the round if a reading fails and sends the temperature as text such as 21.50 °C.
• What is read and not sent. The humidity, the temperature in Fahrenheit and the heat index are calculated and not sent. The function writeSerialToBLE() and the two pin numbers are not used.
• Buffers. The reply of a command is read into a buffer of 100 characters without a check on the length. A longer reply would write past the end of it. Keep the replies short or add a limit.
• Does it reach the phone? The module sends the text to a connected phone, and the app shows it if it listens for notifications on FFE1. The sketch does not check that a phone is connected.

What it should do is what the last version does: set up the module once, then send a value on a fixed interval. The next steps would be to send the humidity as well, to receive commands from the phone, and to use the STATE pin to see whether a phone is connected.

Good to know

• The repository holds the five sketches and is under the MIT licence.

The project is in the Arduino—AT-09-BLE-module repository on GitHub.