I am currently trying to manipulate a multimeter (Zoyi ZT-5BQ) using an Arduino board, but I don't know the protocol that uses my multimeter to change the reading mode (in example, from ohmeter to termometer). I have tried to pair my AT-09 module to the manufacturer's app (Bluetooth DMM) and see what does it sends when I try to change the reading mode from the smartphone, but AT-09 is not detected by the phone, and I guess it's due to the MAC address of my module. Is there any way I can sniff between the communication of my smarthphone and multimeter?
Thanks in advance!
The manufacturers' app can't find the AT-09 most likely because the app searches for devices that advertise a specific service that the module does not offer.
Start your research by installing a generic BLE scanner app such as nRF Connect. Connect to your multimeter and look at the services and characteristics it discovers. Try reading and/or writing from/to them, sometimes this is enough to figure out the easier protocolls.
nRF Connect also offers to debug the connection if nRF Connetc is open in the background and you connect to your device using the manufacturers' app. This can already give some insights to the messages sent and received.
The last resort would be to use a real BLE sniffer. There are multiple options, I personally have great experience using the one from Nordic Semiconductor. You would need a bit of hardware and can use Wireshark with an extension to see everything. The cheapest option for the hardware would be the nRF52840-Dongle.