HiLetgo 2pcs CC2540 CC2541 AT-09 Serial Wireless Module BLE 4.0 Bluetooth Module Compatible HM-10

I have clearly and politely articulated why I (an experienced Bluetooth-er and user of these products) have critical issues with this particular product. Both times those reviews were REMOVED by Amazon. I'm sorry but if AT commands work except for certain critical ones like AT+ROLE (to set Central-Peripheral (master-slave)) this is not HM-10 compatible. Amazon is in a tough spot because I'm sure they don't know who to believe. But what ever. This will probably be the last time I try to warn you folks.

Update! I bought two of these to connect two arduino's together wirelessly and I've got them working. Both of these AT-09 bluetooth modules work fine in slave mode, but it took me a while to get one set up as master. It's challenging because there is no documentation that I have found for exactly this module and the AT commands are different than the ones I found online. It's like a puzzle where you have to guess what the pieces are -- fun right! The magic AT commands are AT+Role11 to switch to master, AT+INQ? to find the slave address, and AT+CONN11 to connect to it. I don't know why you need the extra 1s at the end but that seem to get it to work. Cheers!

Just a heads up, this is a clone of the HM-10, so some of the information you find online regarding the HM-10 will slightly differ. Also, Let me save you the headache of figuring out the AT commands with this module, don't use the "?" character when entering the AT commands. That's it. Otherwise, you'll get an error when trying to enter something like, "AT+ROLE?". I've included a picture of the available AT commands the module writes when entering "AT+HELP."

Not for the faint of heart or beginner. The AT command set for these "MLT-BT05-V4.1" is undocumented. The AT+HELP commands it does have, can ONLY be used for receiving unsecured data from a cell phone as a slave receiver to an MCU. So for $5/each use these for only sending text chars as commands from a cell phone to your MCU. You will also not be able to pair to these from your phone as a normal BLE device. Only an app can connect to them (BLE Scanner, etc.)There are also no AT commands to "bind" or "connect" to another BLE device programmatically from the module on an MCU. So you cannot talk from one MCU to another using these modules. The "State" Pin works to know you have a connection, but the "Break" pin does not work to disconnect, and you cannot disconnect a BLE connection with a reset, only by cutting power to the module.

**Update 4/26/2021**Fixed it David S. So in your IDE serial console, Make sure you have Newline & Carriage Return both selected in the bottom-rightStill undecided how this works - trying to figure out but as David S says there's little to no documentation. By the way David, you saved my hide; I was about to get mad when I realized what was happening.It is some weird carriage return/ASCII encoding error.The device is tricky to figure out but slowly. David you're kinda right. Just use a space afterwards/any character and the AT commands work just fine.