Problem with HC-05 BT Module


I want to use 2 HC-05 BT modules to communicate between 2 Atmega328. The Power Supply is 3.3V.
Originally I wanted to use RF communication, but I am not really happy with this, thats why I want to Switch to the BT module.

Now I tried this Setup with 2 Arduino Nanos and it works perfectly.


Now there is not enough space on my pcb that I already ordered and I have to use only the "small green" PCB of the HC-05 Module.

I connected RX,TX;VCC and GND and also the PIN 34 "KEY" to VCC but now it isnt working anymore.

I also tried this one.

But I cant get it working.
I am not able to enter the AT Mode and communicate with this module (small green PCB).

What am I doing wrong?

I want to test this before I solder everything on my PCB.

Thanks in advance.



I connected RX,TX;VCC and GND and also the PIN 34 "KEY" to VCC but now it isnt working anymore.

That is hardly surprising but, if you simply connect Rx>Tx and Tx>Rx, there is a chance it might work. Failing that, you might go to
and find some help from somebody who actually knows what he is doing. Whichever way you go, I would be rather interested to hear how you fared, particularly if you get a result with the first option. Also, any conclusions you might draw from the experience.

I now added a small LED to the Pin 31 "LED1" on the small green HC05 PCB.
It seems to Enter the AT Mode, but there is one difference.

On the breakout board of the HC-05 the LED goes on and off for 2 seconds when in AT Mode.

on the small green PCB the led goes on and off really fast and then stays on for 2 seconds and then off for 2 seconds. Then again fast on / off and 2 seconds on and 2 seconds off.

I am not sure what the Problem might be.

....... Obviously not too inclined to take any advice then...... I don't know why you bothered asking what you were doing wrong but, you never know, you may discover a nice complicated way that nobody else would ever think of, for solving a simple problem.