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

Project: Trim setpoint reader

Many products hide a trim pot for threshold, brightness or calibration. You will read a pot on GPIO 34 and turn it into a human setpoint display.

Sketch 1 proves 12-bit ADC codes. Sketch 2 is the useful reader: print raw, estimated volts, and LOW/MID/HIGH band for your trim.

GPIO matrix idea

Many peripheral signals can route to a wide set of GPIOs - with exceptions. Trust silkscreen and Espressif notes over Uno D-numbers. Some pins are input-only; some are strapping; some serve flash on modules.

Strapping pins

At reset, classic ESP32 samples GPIO 0, 2, 12, 15 (and often 5). GPIO 0 LOW at reset often enters download mode - useful for flashing, confusing with a casual button.

ADC: 12-bit, attenuation and nonlinearity

12-bit codes 0-4095 with analogReadResolution(12). Finer than Uno 0-1023, but nonlinear at the ends. Power the pot from 3V3. GPIO 34 is input-only - ideal for the wiper. Treat readings as useful, not metrology-grade.

DAC and touch (awareness)

Classic ESP32 DAC on GPIO 25/26; many RISC-V parts differ. touchRead exists on supported pins. This lesson's practical path is the trim pot on GPIO 34.

Wiring and safe build sequence

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

What this sketch is for: Discovery: turn the pot and watch raw codes move through roughly 0-4095.

const int potPin = 34;

void setup() {
  Serial.begin(115200);
  analogReadResolution(12);
}

void loop() {
  int raw = analogRead(potPin);
  Serial.println(raw);
  delay(200);
}

What this sketch is for: Useful calibration tool: print raw ADC, estimated volts, and LOW/MID/HIGH band so you can set thresholds for later lamps and alerts. Match the breadboard layout below before upload.

const int potPin = 34;
unsigned long lastPrint = 0;

const char* band(int raw) {
  if (raw < 1200) return "LOW";
  if (raw < 2800) return "MID";
  return "HIGH";
}

void setup() {
  Serial.begin(115200);
  analogReadResolution(12);
  Serial.println("Trim setpoint reader");
}

void loop() {
  if (millis() - lastPrint < 250) return;
  lastPrint = millis();

  int raw = analogRead(potPin);
  float volts = raw * (3.3f / 4095.0f);
  Serial.print("raw ");
  Serial.print(raw);
  Serial.print("  ");
  Serial.print(volts, 2);
  Serial.print(" V  ");
  Serial.println(band(raw));
}