Thanks for that info.

For anyone else that stumbles across this in the future: what I ended up doing was setting up a simple voltage divider (https://randomnerdtutorials.com/how-to-level-shift-5v-to-3-3v/) and connecting the 3.3V output as an input to one of the GPIO pins, set to pull down.

When the TTGO board is running on battery, the 5V pin I'm using for input from the external power supply becomes a 5V output, which backfeeds into the 5V rail I'm using to detect external power. To solve this, I put a diode between the external power rail and the 5V pin on the TTGO board. This does result in some voltage loss when operating on external power, so I've been advised to use a Schottky diode instead, but I haven't tried that yet.

So far, so good!

@teastain2 Bingo! that worked, changing
Wire.begin();
to
Wire.begin(26, 14);

so pinout for I2C on T-relay 8X is
SDA: pin 26
SCL: pin 14

thank you!

@tierfonorbital : I had the same problem. Couldn't really get an answer anywhere, and the supplied code didn't help.

Eventually I found a working solution. Here's my writeup:

https://sites.google.com/site/3dsfinfo/mini-displays

@windsor With what device are you trying to communicate?

@teastain2,

Thanks again for your help, and for confirming Arduino 2.1.1 was working fine with the LilyGo T-Display S3 on your system.

After spending virtually the whole day on this, I was able to pin-point the problem and work out a solution.

First, let me say I have also been using the ESP-IDF for some projects.

With that in mind, somewhere along the line I updated the CH430/431 driver to its most current version (3.8).

This turned out to have been at the root of the problem. Finding this post: https://github.com/espressif/esptool/issues/879 I tried reverting the driver to version 3.5, but that alone did not resolve the problem - although it was indeed the key part of the solution.

After downgrading the driver, the sketch would still not upload on either of the two LilyGo T-Display S3 boards which I was working with via the Arduino IDE 2.1.1.

However, I found that I could upload a very basic "hello world" like sketch to the LilyGo T-Display S3 using the ESP-IDF. Also, once I did this, I could then upload the sketch that I needed to the same board using the Arduino IDE.

In other words, it appears not only did the use of the current CH430/431 driver (3.8) prevent uploads to the LilyGo T-Display S3, it also somehow appears to have corrupted the flash in the board such that even with the right driver the Arduino IDE could not upload to it.

Using the ESP-IDF to upload to the board, seems to have reset things such that the Arduino IDE would again work fine with the 3.5 version of the CH430/431 driver.

I repeated the process with my second LilyGo T-Display S3 and got the exact same positive results.

Just posting this here in case it will be of use to others.

Thanks again to @teastain2 on this forum, and xamfa, radimkarnis. and mentesix on the Espressif forum for there help and posting toward this resolution.

@akira Great!
Thanks for replying!
-Terry

@zendiego Here is the hard-core technical site for your product:
https://github.com/Xinyuan-LilyGO/T-Echo
There are example Arduino IDE sketches and a full schematic!
If you are not a hobbyist-programmer, I think you should seek your answers at the Meshtastic group.
-Terry

I did it

https://www.youtube.com/shorts/PqeYRstlqXU

I couldn't add it to the t-echo so I did it separately
up sound always on, down sound only < -2m/s

The idea was to be able to put a sound on the lilygo t-echo, after trying to get the programming code and not being successful (open source, but I couldn't find it). I decided to make a separate arduino (I know it could be solved by adding the buzzer to the t-echo, but without the program it is unfeasible). arduino variometer project that has on the internet! The idea is to use t-echo on the Kobo Touch with XCsoar, as the kobo doesn't have sound the way I got it was this! Now I just have to learn how to put a bluetooth module on the kobo, how to pair the hc05 with the t-echo (if anyone knows or can give me a tip, I'd be grateful) imagine getting sound and another screen like that on the lilygo t-echo would be the dream https://www.youtube.com/shorts/B1Fd2gZ12Ms

@bpupp This is how it is done in Arduino!
https://www.arduino.cc/reference/en/language/functions/communication/serial/read/
Not sure this is helpful, but may give you some insight to convert to ESP-IDF!

thanks for your reply !