Low voltage in 3.3V operation Rev 3 board

[yvdh]

Hello again.

Back in 2020 I never got my boiler (Remeha Quinta Solo) to work with the OpenTherm shield Rev 3 : as soon as I connected it the boiler would start working full tilt. This is in 3.3 Volt operation, with a Sparkfun ESP32 Thing board. The hardware tests seem to work, but I didn't try the software tests.

I though this was a boiler issue, and dropped the project.
Recently I picked it up again and upon measuring the shield I find that the output voltage for your test #4 only reads 14.5 V instead of 15 - 18 V. According to OpenTherm specs this is too low to represent the 'high' state. Also, the high state for the MASTER IN pin only goes to about 1.78 V, which is too low to reliable represent logic hi for 3.3 V.

I tried modifying R11 from 4.7 K to 6 K, but this only increases the MASTER IN level to 2.4 V or so. Moreover, the board does not seem to work properly anymore. I also noticed that R9 which should be 39 Ohm is almost 50 Ohm which is quite a lot off .... My electronic skills are not good enough to tell if this should have a big impact or not.

I know you have a Rev 4 board, but I have already purchased 2 boards (the first one was a self assembly and did not work at all but that was surely my fault).

At this point I don't want to buy another board, more likely thinking of reverting the OT board to 5 V operation and using a 4 channel logic level shifter (~5 euro) to interface with the ESP32.

Any suggestions or insights are appreciated.

Cheers,
Yves