Two Unos over a pair of HC-05s so the one with the GSM modem reports both

Solved 2 3

Posted October 23, 2026 at 10:06 am #1

Home project. Uno A in the shed reads soil moisture for the greenhouse beds and a door contact. Uno B in the house has a SIM800L GSM module (no broadband at the house yet, long story). B should forward A’s readings to Hermetry together with its own. The shed is about 15 m away so I’m trying two HC-05s, A as slave and B as master.

Configured in AT mode:

slave (A):   AT+ROLE=0   -> OK
             AT+ADDR?    -> +ADDR:98d3:31:fb1234
master (B):  AT+ROLE=1   -> OK
             AT+CMODE=0  -> OK
             AT+BIND=98d3:31:fb1234  -> ERROR:(1D)

The BIND fails, and when I tried CMODE=1 instead the master LED keeps blinking fast forever and never connects. What am I doing wrong?

Posted October 28, 2026 at 12:29 pm #2

Fast blink means not connected. Once they link you get two quick blinks every couple of seconds.

Posted October 29, 2026 at 4:47 pm #3

I did exactly this for a uni lab last term and got the same ERROR on BIND. The address comes out of AT+ADDR? with colons, but AT+BIND wants commas, in the order NAP,UAP,LAP:

AT+BIND=98d3,31,fb1234

Full sequence that worked for us, both modules entered with KEY/EN held high at power-up (AT mode is 38400 baud):

slave A:  AT+ORGL
          AT+ROLE=0
          AT+PSWD="4321"
          AT+UART=9600,0,0

master B: AT+ORGL
          AT+ROLE=1
          AT+CMODE=0
          AT+PSWD="4321"
          AT+BIND=98d3,31,fb1234
          AT+UART=9600,0,0

Same PSWD on both, then power-cycle both without KEY held. Master pairs within a few seconds. After that it’s a plain 9600 serial line between the Unos.

You must be logged in to reply to this topic.

Join the conversation

Sign in to ask and answer, like helpful posts and mark the reply that solved your problem. Every contribution earns points and levels.