|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.|