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After asking How to connect Arduino & ESP8266, I tried to build such circuit.

First I tried the code one 2 Nano's - to verify 2 MCU sending and receiving messages. After that, I started to build the following circuit:

Take1: MCU A: Arduino Micro 5V, MCU B: WEMOS mini. R1=10K, R2=20K. FAIL

Take2: MCU A: Arduino Micro 5V, MCU B: WEMOS mini. R1=1K, R2=2K. FAIL

Take3: MCU A: Arduino NANO ,MCU B: WEMOS mini. R1=1K, R2=2K. FAIL

Take4: MCU A: Arduino NANO 5V, MCU B: WEMOS mini. Tx to Rx (no vdiv ). OK

Updated diagram

Edit1: LCD display is connected to MCU B ( as it was tested using 2 Nano ) via I2C. Messages received are sent to LCD. PLEASE IGNORE LCD shown in not I2C

Edit2:

  1. Diagram updated - GND was missing and POWER IN clarification.

  2. LCD display is an I2C one. It shows properly WEMOS is booting up properly ( shows a boot msg ), and spits every incoming Serial message from other MCU. When TAKE4 and TAKE1 were taken, setup and code worked PROPERLY.

What is done wrong?

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    In your wiring diagram you are missing the ground connection to the Wemos. Also you have the 5V pin of the Wemos connected to Vin on the Nano, I guess to provide the Nano with power. Thats incorrect. 5V needs to go to the 5V pin. Vin is connected to the internal voltage regulator, which needs at least 6V to power the Nano. – chrisl Apr 18 at 19:40
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    Your frizzy thing does not help much, a schematic is better. Use a level converter, doing the resistor trick is playing with the margins of the devices and is not reliable. They give voltage ratings and threshold levels for a reason and it is good to stay away from the margins, the closer you get the more susceptible to noise you are. – Gil Apr 18 at 20:34
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    @Majenko yes, it is an error. I'll update the diagram – Guy . D Apr 19 at 11:22
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    You seem to be mixing Nano and Micro boards in your setup. You do know, don't you, that the Micro has a 32U4, so Serial is the USB but not the TX/RX pins - you need Serial1 for those. Also the TX/RX pins on the nano are connected to the FT232 chip, which will most likely interfere with your voltage divider. – Majenko Apr 19 at 21:19
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    Guy.D, yes use Serial1 for communication with esp8266 on RX/TX of the Micro – Juraj Apr 20 at 8:38

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