JDY-33 bluetooth module does not work with hardware RX pin of Arduino UNO

Projects Networking, Protocols, and Devices
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This post is actually a note for the future searchers, since other similar topics have already been closed:

Brief description of a problem:
If one connects a JDY-33-like bluetooth module to hardware RX/TX pins of Arduino UNO or Nano, the data can be transmitted from Arduino to JDY-33 module successfully, but not in the opposite direction.
In other words, the JDY-33 module understands what Arduino sends to it, but Arduino does not understand what JDY-33 module tries to send in reply (even if the TX LED blinks on the board!).

I believe I have found an answer to the question of why this is happening (and also, why this does not happen with the HC-05-like modules (reportedly)).

The reason seems to be the presence of the on-board USB-to-serial chip (e.g. CH340G) connected to the very same RX and TX pins of the microcontroller. It seems the JDY-33's TX pin is just too weak to pull down the TX line of the USB-to-serial module. Therefore it was reported to work fine with SoftwareSerial, but not with the hardware serial interface (with USB-to-serial chip attached).

Another symptom is that this module works perfectly fine with the hardware RX/TX of Arduino Pro Mini which does not have an on-board USB-to-serial chip. It does also work well with any USB-to-serial adapter attached to a PC.

So I tried the following rather naïve schematic to adapt the JDY-33 module to Arduino Uno. The idea is simply to put a PNP transistor in between, with its base connected to JDY-33's TX pin, and its emitter -- to the RX pin of Arduino Uno board like this:

Arduino_JDY-33_bluetooth_connection_with_divider
Arduino_JDY-33_bluetooth_connection_with_divider1465×1431 212 KB

I am really not sure this is a correct schematics, but afther this modification, the JDY-33 module started to talk to Arduino Uno absolutely fine in both directions, and does not even require to disconnect the module for programming.

A small note: I actually use some no-name replica of Arduino Uno, which, although works at 5V itself, applies 3.3V to its TX line, so there is no need in voltage divider in my case. And I doubt this simple connection of a transistor would work for 5V TX level.

I do also believe my findings may be a possible answer to this question on the forum:

Arduino NANO and Bluetooth modul - why not use Rx Tx pins?

1 Like
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Could you please elaborate on this.

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Thanks for drawing attention to this point. It looks like I just did a wrong measurements at some point in the past, and than relied on this wrong result. I do not have a proper oscilloscope, so I am unable to watch it in action, but I have just measured the TX pin in idle state with a multimeter, and it does show 5V. So I updated my original post to reflect this mistake.

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OK
You are probably aware that there is a 3.3v ProMini, so I have wondered about a 3.3v Uno.....

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