Phantom Power problem

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Phantom power problem
Phantom power problem1920×1440 147 KB

I have an Arduino Mega controlling a few sensors and communicating with a few other devices (some not shown). It is powered with 5v from a lipo battery (charged by solar panels). It is connected to my PC (not powered by) for serial communication, and also has serial communication with an Arduino Micro, which itself is connected to the PC (not powered by) to act as a keyboard/mouse.
This all works fine (kinda - see problem 2 below) for now, but i now want to add an ESP32 device. However, the ESP is very power hungry, so i can't afford to power it through my solar charged battery, i have opted to just power it by the mains supply, using 5v, 1A adaptor.
Problem 1: Now i have added the ESP everything works fine, BUT when i turn off the power switch from the battery, the LCD screen still faintly shows what was being displayed before it was turned off. So the LCD is getting powered through the TX2 to RXSS line from the ESP? This doesn't sound good, what is the best way to solve this? (note, all other hardware serial ports on the MEGA are being used, i can only communicate with the ESP over SoftwareSerial, not negotioable! )

Problem 2: a few weeks ago, before i tried to add the ESP, i tried to add a circuit that measured the voltage of the battery, this is a wire from A5 to the positive side of the battery through a 10k ohm resistor. I had to remove this circuit because it was giving me wrong readings (even though i tested the code successfully on a simpler circuit), i think the Micro was being affected by it somehow also, is it possible am i also disrupting the reading by having a serial com with the Micro??

note: RXSS / TXSS = Software serial lines

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A LCD display remembers its last state for some time. Visibility may be a matter of background illumination, depending on LCD type.

Measure the current (voltage) across that resistor. False ADC readings also can come from varying supply voltage of the controller when used as analog reference AREF.

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Thanks for the reply! I'm using a 1.3 Inch 128X64 I2C OLED Display Module. Before i added the ESP32 to the circuit, when i would turn off the battery power the display would turn off/ go black straight away, but now when i have the ESP32 in the circuit and powered by the mains, when i turn the battery off the mega turns off, but the display faintly remains for more than 5 minutes (i havn't tested any longer i'm worried it might be damaging something). So somehow power is getting from the mains supply into the mega?? through the TX2 to RXSS line from the ESP32?

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You should never connect powered ICs to unpowered ICs through I/O pins, as that can end up destroying one or both. To save power, use sleep modes instead (the ESP32 has them).

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