An ESP32 and an Arduino Uno talking over serial: an ESP32 DevKit and an Arduino Uno

// ESP32: reads a potentiometer on GPIO 34 and sends the reading to an
// Arduino Uno as a line of text, four times a second.
//
//   ESP32 GPIO 4 (TX)  ->  Uno pin 2 (RX)
//   ESP32 GND          --  Uno GND
//
// One way only, on purpose. The ESP32 drives 3.3 V, which an Uno reads as
// HIGH. The Uno drives 5 V, which a real ESP32 pin is not rated for, so a
// link back would need a divider or a level shifter on that wire.

#include <SoftwareSerial.h>

SoftwareSerial link(-1, 4);  // no RX, TX on GPIO 4

void setup() {
  Serial.begin(115200);
  link.begin(9600);
  Serial.println("ESP32: sending the knob to the Uno");
}

void loop() {
  int knob = analogRead(34);  // 0 to 4095 on the ESP32's 12-bit ADC
  link.print("knob ");
  link.println(knob);
  Serial.print("sent knob ");
  Serial.println(knob);
  delay(250);
}

// Arduino Uno: reads "knob N" lines from the ESP32 on pin 2 and turns the
// LED on pin 9 up and down to match, with PWM.

#include <SoftwareSerial.h>

SoftwareSerial link(2, 3);  // RX on pin 2 (pin 3 is not used)

char line[24];
byte len = 0;

void setup() {
  Serial.begin(9600);
  link.begin(9600);
  pinMode(9, OUTPUT);
  Serial.println("Uno: listening to the ESP32");
}

void loop() {
  while (link.available()) {
    char c = link.read();
    if (c == '\n') {
      line[len] = 0;
      len = 0;
      if (strncmp(line, "knob ", 5) != 0) continue;
      int knob = atoi(line + 5);
      // The ESP32 counts to 4095; analogWrite takes 0 to 255.
      analogWrite(9, map(knob, 0, 4095, 0, 255));
      Serial.print("Uno got knob ");
      Serial.println(knob);
    } else if (c != '\r' && len < sizeof(line) - 1) {
      line[len++] = c;
    }
  }
}