The schematic is here.

Bonjour, Bravo pour ce projet. Je serait interessé egalement par le sources du projet et les caracteristiques de differents modules. Comment puis-je vous contacter ? Merci d'avance.

Hi, Bera! how are you? Did you get to try out the draft 4" TFT Analog-GPS clock on Arduino? It's been a while since I tried to use it with the Display 3.5 inch ILI9488 with esp8266 Nodemcu or esp32 but it did not work (showed some errors). Have you tried or updated the sketch for ESP? You told me about these other two projects: Youtube link1 & Youtube link2 but, I prefer the 4" TFT Analog-GPS clock on Arduino. If you have something updated and you want to send me, I thank you. If you prefer, my email is: meuviolino@hotmail.com Note1: I have both 3.2 inch ILI9341 and 3.5 inch ILI9488. Note2: I had sent you, attached, the photo of my display: TFT ili9488 3.5" (1.6 MB) - I think you did not see it. Thank you very much, Daniel Fernandes

Upto ~200V the ADC reading and the actual voltage at the terminals follows the selected voltage accurately (some program tweaking was required accounting for component tolerances). That’s why I decided not to mess with the DAC (would require a non-linear adjustment function). I can select 400V output and will actually get 393V max. output (ADC reading and at the terminals). My main goal was to get the ADC reading accurately inline with the voltage at the terminals over the whole range of voltage supply within +- 0.5% accuracy. Add: i couldn’t sleep and fixed the DAC as well. I get 400V now. ADC, DAC and voltage output at the terminals all now within +-0.3% (1V) accuracy. Good enough...   I carefully selected components, especially Oamps resistors and did the fine tuning via software modification, mainly applying linear regression curves of the form y=a*x + b to calibrate output of the read_ADC, check_Anode_supply and check_Grid2_supply functions. Grid1 and heater are ok. That’s it for me. I Will probably use and apply this concept for other voltages and currents for general purpose programmable power supply’s...

Agreed.   An update from me -- I ordered a second MEMS microphone BoB and with this one the setting of P2 worked as expected (albeit only changing behaviour at about one turn above 0).  However, with this second microphone board  (and a third which I wanted to put on a longer lead) I did not add the decoupling capacitor on the microphone board. So my current hypothesis is that the MEMS board with decoupling capacitor is just "not noisy enough" for the P2 setup to work .   I'm just off on a trip where I'll be using the detector in caves, but when I get back in a few weeks I'll try adding the decoupling capacitor to the 'working' MEMS board and see how that affects the circuit. Very many thanks to all who posted here, and to Ken Horton who suggested getting a second MEMS board.   At present I have a device that works as described in the article. Hmm .. wonder if a Rev3 might be in order?  Single-rail op-amps to avoid the need for level shifting; maybe more done in software;  maybe stereo .... ?