Update hardware schematic to include powerlevel shifter for i2c bus

[ableischwitz]

I tried to re-build the schematic as shown in the diagram.
Due to the 3.3v from the ESP i2c bus and the display was working as expected, but as soon as the 5v operated NFC reader got added, the display behaved very weird.
I had to add a level shifter like shown here in order to reliable read NFC tags and also have a stable display.

For anyone also suffering from a non functional NFC reader: try adding a level-shifter.

| GND | GND     |
| HV  | 5v      |
| LV  | 3.3v    |
| HV1 | SCL NFC |
| HV2 | SDA NFC |
| LV1 | SCL ESP |
| LV2 | SDA ESP |

I didn't check whether the display or the NFC has i2c termination enabled - in any case it would usually be required to have this on the bus.

[ManuelW77]

Strange, i've never had this problem with my 3 own scales.
But fine if it work for you now :)

[ableischwitz]

While it may work, the current setup without level shifter is working due to stretched tolerances.
By design a high level is 0.7 * Vdd - which is 5*0.7= 3.5 for the NFC reader. The ESP will only be able to provide 3.3v at best - and depending on the tolerances of the reader a high-signal may be identified as such - or not.
See https://www.nxp.com/docs/en/user-guide/UM10204.pdf Section 3.1.2
Even though you may not see issues with your setup, I have seen several reports of NFC failing to work. Some of those issues may be reasoned due to the lack of proper I2C level.