Even after level-shift SIM800L RX-TX not connecting to ESP32

Hardware
1

I am trying to connect SIM800L to ESP32 DevkitC via serial but it wasnt working (I could not see the result of any AT commands). SIM800L connects to network and I can call it just fine ( blink after 3 seconds). It is powered separately via a buck converter and a decoupling system with a zener diode.
ESP32 is powered via USB as I am monitoring the program output via Arduino IDE.

I noticed that SIM800L TX output was hovering around 1.3V to 2V so I tried to level shift it to 3.3V so now 3.3V appears at RX pin of esp32 when SIM800L gives logic 1, and floating value when SIM800L gives logic 0 (as checked by my voltmeter).

But, I still dont see any response to AT commands in my serial monitor. I have tried the same SIM800L with another esp32 (devkit-1), and with nodemcu esp8266. But nothing on serial output after AT commands.

Here is my schematic:

sim800levelshifter

Here is my code:

unsigned long previousM = 0;
const long resetBurst = 15000;

void setup() {
  Serial.begin(9600);
  Serial2.begin(9600);
  Serial.println("Starting");
 
  delay(3000);
  ReceiveMode();
}

void loop() {
unsigned long currentMillis = millis();  
if(currentMillis - previousM >= resetBurst){
previousM = currentMillis;
Serial.println("Getting in Receive Mode");
ReceiveMode();
}
}

void ReceiveMode() {
  Serial.println("Inside Receive Mode");
  Serial2.println("AT\r");
  updateSerial();
  Serial2.println("AT+CGSN\r");
  updateSerial();  
  delay(3000);
}

void updateSerial() {
    delay(500);
    while (Serial.available()) {
  Serial2.write(Serial.read()); 
   }
    while (Serial2.available() > 0)
    {
Serial.write(Serial2.read());
}

The output I see in serial monitor is just this:

Getting in Receive Mode
Inside Receive Mode
Getting in Receive Mode
Inside Receive Mode
Getting in Receive Mode

What am I doing wrong?

2 
Serial2.begin(9600, SERIAL_8N1, 16, 17);

What I found is that the SIM800L need voltage between 3,17 and 4,4V. Not 5V.

As level shifter you can better use TXS0102. A floating value is bad.

3

My apologies. I wrote it wrong, the buck converter is actually giving 4.2V to SIM800L not 5V.

I tried appending Serial2 with explicit pin numbers but same result.

4

I have couple of units , both having ESP32 and SIM800L powered off of a buck with output adjusted to 3.5V ( 100uF 10V electrolytic cap at the output )

and they both just fine with no level shifter at all

5

I had one too that didnt need any level-shifter but without a decoupling circuit at VCC it blew up after a few days due to a current burst from a 470uF cap.

6

Ran I2C Scanner with pins 21 and 22 connected to TX and RX of SIM800L pulled up by 10k resistors but nothing. It cant even find the device.

7

Update: tried two BCM 547 transistors and took the output of the second transistor's collector and gave it to ESP32's RX. It shows 3.27V constantly (just as TX from SIM800L shows -2.23V constantly) but ESP cannot read it. On serial monitor response to AT/r is not shown.
Can anyone help?

8

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