When is an HC-05 module not an HC-05 module?
When it’s a BLE module in disguise, running firmware hc05V2.3_le.
Introducing the HC-05, Firmware Version hc05V2.3_le:
This version seems to have appeared with no announcement, fanfare, datasheet… or any sort of differentiation from any of the actual HC-05 compatible modules on the market. So far as I can gather from what little information I can find on this new variant of the HC-05, it started appearing around summer 2024, and confusing hobbyists with its non HC-05 like behaviours, despite being sold as, and presenting itself on Bluetooth scans as, an HC-05 module. I’ve not seen a single seller differentiating these from other HC-05 compatible modules – which I’ve fallen foul of!
What’s Different?
The main difference:
The HC-05 is a bit of a legacy Bluetooth device, utilising the Serial Port Protocol (SPP) of Bluetooth 2.0. The new module, which I’m referring to by it’s firmware version hc05V2.3_le utilises Bluetooth 4.0, which does not have a built in SPP, and cannot communicate with the standard HC-05, or devices/apps designed to work with it.
This makes it completely incompatible as an HC-05 replacement.
Other observed differences:
– The Bluetooth name, as observed when scanning for it from a phone or computer, is all in capital letters.
– It automatically enters AT mode for a short period of time after power on. I’ve not managed to measure this, but in the region of 200ms.
– To enter full AT mode, the button needs to be pressed before power is applied, then remain pressed to maintain AT mode
– It requires a 6 digit connection code, instead of the standard HC-05 4 digit.
– When queried with ‘AT+VERSION?’ it responds with ‘hc05V2.3_le’, instead of one of the recognised HC-05 firmware versions.
How can I tell if I’ve got one?
Firstly, compare it to the picture at the start of this post! The differences can be very subtle – the placement and size of the main three components, the two ICs and the crystal, can be very similar. This V2.3_le module has some small passive components (Two caps and a resistor, plus some unpopulated footprints) next to the crystal, and a different layout of unpopulated footprints between the smaller IC and the antenna.
The HC-05 compatible module and an V2.3_le for comparison:
In use, any name you set via AT command will appear in all capitals, and if you connect to the module using a Serial Terminal App such as ‘Serial Bluetooth Terminal’ by ‘Kai Morich’, the device comes up in the Bluetooth LE tab, instead of the Bluetooth Classic tab.
For those of you with an eChook nano board, who I imagine might be the majority of my tiny audience, and are using an Arduino Nano Every, the latest code in the main branch of https://github.com/eChook/eChook-Arduino-Nano now prints out the firmware version of the HC-05 module during the configuration routine.
For everybody else, the relevant snippet of code is below – adapt the serial ports to make sense.
//HC-05 on Serial1, Terminal on Serial
// Ensure HC-05 module is in AT mode when you run this.
Serial1.print(F("AT+VERSION?\r\n"));
Serial1.flush(); //Wait for transmission to end
// Now Check Response
delay(50);
String response = (Serial1.readStringUntil('\n'));
response.trim();
Serial.print(F("HC-05 Version: "));
Serial.println(response);
Can I avoid buying one?
Not easily it seems. I’ve received these twice now, the first time, just a few mixed in with a batch of actually HC-05 compatible modules, and the latest time three full trays of them, being sold (probably completely innocently) as HC-05 compatible modules, where the listing picture does not match the hc05V2.3_le modules I’ve received.
The AZDelivery HC-05 modules on Amazon have recently proven to still be HC-05 Compatible modules, but I obviously can’t vouch for that in the future. I’d aim for listings that specify Bluetooth 2.0, so that you have an objectively valid reason to return it if a hc05v2.3_le module arrives.
If anyone has any more information on these new modules, please leave a comment!