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I have a HC-06 bluetooth module. The module has an RX and TX pin (the RX pin receives data and TX transmits data) which connect to the TX and RX pin of an Arduino respectively (in my case an Arduino pro mini).

My question is in what scenario would someone use the RX pin of the module(basically for what purpose would the bluetooth module receive a signal from the Arduino)?

For example let's say i have a simple LED that i turn off and on. I just sent the signal from my phone to the module and the module sends the signal to the Arduino. So basically i've connected the TX pin of the module (which transmits the data) with the RX pin of the Arduino (which receives the data).

In order to connect the RX pin of the module with the TX pin of the Arduino i have to use a voltage divider as the module only accepts 3.3 volts of signal while the Arduino transmits 5 volts of signal. I want to avoid that because in my project i need the PCB to be quite small.

So if someone only wants to transmit data from the module to the Arduino, is there a reason on why would he connect the other two pins?

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If you never want to change the configuration of the module either, then no, you don't need to connect the module's RX pin.

However many people want bi-directional communication. Not only the ability to send a signal to turn the LED on or off, but also the ability to get the current state of the LED - is it on or off?

Or maybe to send an analog reading. Or the data from some sensor.

The HC-06 is a bi-directional module. If you choose to use only one direction then you only need to wire in one direction. However, tie the RX pin to 3.3V with a 10KΩ resistor to stop it floating and consuming extra power or causing random noise to be transmitted to your phone.

In fact, it's just like a phone. You can call someone up and just speak to them without listening to what they are saying. That's fine if all you want to do is call Mom and say "Pick me up in 5 minutes" and hang up. But if you want to ask your Mom "Can you pick me up in 5 minutes?" you have to actually listen for her answer "No, walk home you lazy oik!".

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