E04. Pinout, Strapping Pins, ADC, DAC and Touch

What you will do

Find out which pins are safe to use, then read a potentiometer (pot).

Sketch 1 prints the pot reading, from 0 to 4095. Sketch 2 turns that number into a word: LOW, MID or HIGH.

Use the GPIO numbers, not Uno numbers

ESP32 pins are named by GPIO number, such as GPIO 2 or GPIO 34. Use the numbers printed on your board, not Uno names like D7 or A0.

On a classic ESP32 two groups need care. GPIO 34 to 39 can only be inputs - fine for a pot, no good for an LED. GPIO 6 to 11 are used by the board's own memory chip - never connect anything to them.

Start-up pins (strapping pins)

When the ESP32 starts, it checks a few pins to decide how to start. These are called strapping pins. Wiring a button or sensor to them can stop the board starting or uploading.

Reading a pot: 0 to 4095

The Uno's analogRead gives 0 to 1023. The ESP32 gives 0 to 4095, so it can tell apart smaller changes. analogReadResolution(12) makes sure you get the 0-4095 range.

Power the pot from 3V3, not 5 V. Readings near the very ends of the turn are less accurate - that is normal.

Use GPIO 32 to 39 for anything you read while Wi-Fi is on. Many other pins stop reading when Wi-Fi is running.

Touch pins and true analog output

Some ESP32 pins can feel a finger: touchRead(4) gives a smaller number when you touch a wire on GPIO 4 - try it in the challenge. A classic ESP32 can also output a real voltage between 0 and 3.3 V on GPIO 25 and 26 (the DAC). The course does not need the DAC.

Wiring and safe build sequence

1. Potentiometer outer legs -> 3V3 and GND; middle leg (wiper) -> GPIO 34
2. USB to DevKit; Serial Monitor 115200

What this sketch is for: Turn the pot slowly from one end to the other and watch the numbers.

const int potPin = 34;                 // the pot's middle leg is on GPIO 34

void setup() {                         // runs once
  Serial.begin(115200);                // start Serial at the ESP32 speed
  analogReadResolution(12);            // readings will be 0 to 4095
}

void loop() {                          // runs over and over
  int reading = analogRead(potPin);    // read the pot
  Serial.println(reading);             // print the number
  delay(200);                          // slow the printing down
}

What this sketch is for: Split the turn of the pot into three bands and print a word for each. Match the breadboard layout below before upload.

const int potPin = 34;                 // the pot's middle leg is on GPIO 34

void setup() {                         // runs once
  Serial.begin(115200);                // start Serial at the ESP32 speed
  analogReadResolution(12);            // readings will be 0 to 4095
}

void loop() {                          // runs over and over
  int reading = analogRead(potPin);    // read the pot
  Serial.print(reading);               // print the number first

  if (reading < 1365) {                // bottom third of the turn
    Serial.println("  LOW");           // print LOW after the number
  } else if (reading < 2730) {         // middle third
    Serial.println("  MID");           // print MID
  } else {                             // top third
    Serial.println("  HIGH");          // print HIGH
  }

  delay(250);                          // slow the printing down
}