This is the first prototype of a RS232 / Bluetooth adapter. Its design is quite simple:
Use a standard MAX232 circuit and connect the ttl out/inputs with an HC05/06 bluetooth transceiver.
Although some MAX232 seem to work with 3,3V I had bad experiences with it so I drive the MAX with 5V and the HC05/06 with 3,3V.
2 bridge diodes reduce the 5V to about 3,3V to power the HC05/06. A level shifter transforms the TTL signals between 5V and 3,3V. MAX would understand 3,3V, but the HC05/06 most likely will sooner or later die from the 5V.
In this picture of the prototype the level shifter is missing.
Since the RS232 does not have a power pin, the 5V have to come from somewhere else. Usually there is a USB connector around => take it from there (see the grey little cable go away from the board, thats the link to the USB power).
Then I bought this in the internet.
I had a lot of touble with it. No manual available so I had to search the whole internet for info about it. First I attached a Mini USB to power it, easy. A red LED indicated operation and a blinking blue LED indicated the will to pair.
Pairing was also not really hard to do. Device could easily be found and be paired with my mobile and a tablet.
Then the trouble started. The BT connection was lost latest after 2 minutes. All the time. It drove me crazy. As experiment I unpluged the USB power and used the BAT connector to provide it with 5V. Wow. No connection losses anymore.
Next I had to find out how to increase the default of 9600 Baud. In order to do so it is as it is with all HC06: Connect a TTL Terminal to the HC06. For that there are four PCB Pads you have to provide a 4pin hreader row ( you can see it in the picture between the HC and the switch).
After the Baud change was done the real life test with a RS232 device had to be made. That is where I currently am. In principle it works, but even with 115200 Baud it is slow and has lags. I had this effect with other HC05 “compatible” transceivers as well. Well, I will hang on to that.
Would I buy it again? No ! I am thinking about getting my prototype to a PCB and make 4-5 of it.
What do you need RS232 interfaces anyway? Good question: MY purpose for them is to control Measurement devices (like from “Rigol”).