🛠 Toucher capacitif : touchRead sans composant

Objectives

Use the RA4M1's capacitive touch peripheral (CTSU): 14 pins on the board can serve as capacitive touch sensors, read by a simple touchRead call from the official CapacitiveTouch library. You will calibrate a threshold by observing actual values, then trigger an LED matrix display — a button without a button, without resistors or wires other than a piece of conductor.

Hardware

• 1 Arduino UNO R4 WiFi + USB-C cable
• 1 bare connecting wire (the "pad": a piece of wire, a copper plate, or glued aluminum foil)
• Optional: a second wire to compare two touch zones.

Complete Code

#include <CapacitiveTouch.h>

const uint8_t PAD = A1;      // une broche compatible toucher
const int SEUIL = 100;       // à calibrer avec le Moniteur série !

void setup {
  Serial.begin(115200);
}

void loop {
  int v = touchRead(PAD);    // ~0 au repos, monte franchement au toucher
  Serial.println(v);
  if (v > SEUIL) {
    MatrixSmiley;          // votre fonction d'affichage (voir tuto matrice)
  }
  delay(20);
}

Expected Verification

At rest, touchRead displays a low and stable value; as soon as a finger touches the bare wire (or the board near the relevant pin), the value rises significantly above the threshold. Fine-tune the threshold to the midpoint between your two measurements: this is THE correct calibration method, identical to that of an analog sensor.

Common Errors

• Choosing a non-touch-compatible pin: touchRead then returns a frozen value — consult the official schematic beforehand.
• Setting the threshold "by guesswork" at 50 or 500: without prior measurement, the sensor never triggers or triggers constantly (humidity, long cable acting as an antenna).
• Touch cable too long without a finger or insulation: a 30 cm wire picking up 50 Hz interference makes the value fluctuate; keep the pad short or surround it with adhesive tape.
• Confusing it with an analog reading: touchRead does not measure voltage, but the variation in capacitance caused by the finger; a voltage divider on the pin is unnecessary.
• Forgetting #include <CapacitiveTouch.h>: the library is not included by default in all sketches.