Bluetooth modification for Ferrograph Aurora 63

Some time ago I got a Ferrograph Aurora 63 LED display. Its a 16x135 matrix of dual color (red/green) LEDs. It has the custom XDF firmware installed, which allows it to be controlled using the well documented Alpha Sign protocol. The commands are given to the display through either RS422 or RS232.

Figure 1. The panel displaying a message on my work bench.

As RS232 serial ports are quite rare today, I decided to modify the display to use Bluetooth instead. The display uses a MAX232 level shifter for the RS232 communication. That is, to shift the levels between the higher voltage RS232 levels and the TTL compatible logic levels which the processor of the display (Z180) uses. My idea was to make a Bluetooth module, which would act as a drop-in replacement for the MAX232 chip.

I have some HC-05 Bluetooth serial adapters on breakout boards (ZS-040) from eBay, which I planned to use for this project. These modules use 3.3V CMOS logic levels. The output from one of these can be directly fed into the TTL compatible input of the Z180 and it will function properly. However, the input pin on the Bluetooth adapter is not 5V tolerant, and the output from the Z180 would damage it. I used a simple resistor divider (12kohm:22kohm) to bring the voltage down from 5V to 3.3V.

Figure 2. The resulting module. I used horrible phenolic paper prototype board.

Figure 3. Module installed in place of the MAX232 chip. It is difficult to open the thing up for a better photo, sorry.

I was worried that the aluminum chassis of the display would block the radio signals, but this thing works all the way from the other side of the house. Now to just mount it on the wall and write some software to display useful information on it :-).

Update 2020/05/17

Some people have asked for the schematic for my MAX232 replacing module. I didn't publish it here originally, as I didn't have one. The thing was just improvised at the time. I've since drawn the schematic, so here it is.