Raspberry Pi Touch Switch

// Filename: touch_toggle.c

/* Compile with gcc -lwiringPi touch_toggle.c -o touch_toggle
 * Run with sudo ./touch_toggle
 * Do not touch while starting the program so it can initialize properly
 */

/* SCHEMATIC
 *
 * ,----------------------,
 * |   Raspberry Pi       |
 * |                      |
 * | TOUCH_PIN        VCC |
 * `-----+-------------+--'
 *       |             |
 *       +---[1MΩ]-----+
 *       |
 *   Touch surface
 *
 */
#include <wiringPi.h>
#include <stdio.h>
// Note: Pin numbers are in BCM notation (pin number format is set by wiringPiSetupGpio)
// See pinout.xyz
#define TOUCH_PIN 20
#define OUTPUT_PIN 21
// How long to pull the touch pin low
// Controls loop speed and affects CPU usage
#define DELAY 15

int main(void) {
    wiringPiSetupGpio();
    unsigned int timer;
    unsigned int threshold = 0;
    unsigned char state = 0; // Currently being touched?
    unsigned char out_state = 0; // State of output pin
    signed char hysteresis = 0; // Counter for consecutive readings
    pullUpDnControl(TOUCH_PIN, PUD_OFF); // Not sure if this would ever be set, just to be safe
    pinMode(OUTPUT_PIN, OUTPUT);
    digitalWrite(OUTPUT_PIN, out_state);

    // Measure capacitance to calibrate touch sensitivity
    for (char i=0; i < 10; i++) {
        // Pull touch pin low to discharge
        pinMode(TOUCH_PIN, OUTPUT);
        digitalWrite(TOUCH_PIN, LOW);
        // Wait a bit
        delay(DELAY);
        // Start timer
        timer = micros();
        pinMode(TOUCH_PIN, INPUT);
        // Wait for pin to become high
        while (!digitalRead(TOUCH_PIN));
        // Get time elapsed
        threshold += micros() - timer;
    }
    // Set threshold to twice the average capacitance
    threshold /= 5; // This number might need to be increased if the touch is not sensitive enough
    printf("threshold=%d\n",threshold);

    while (1) {
        pinMode(TOUCH_PIN, OUTPUT);
        digitalWrite(TOUCH_PIN, LOW);
        delay(DELAY);

        timer = micros();
        pinMode(TOUCH_PIN, INPUT);
        while (!digitalRead(TOUCH_PIN));
        timer = micros() - timer;

        if (timer > threshold) {
            if (hysteresis < 0) hysteresis = 0;
            hysteresis++;
        } else {
            if (hysteresis > 0) hysteresis = 0;
            hysteresis--;
        }

        // 3 consecutive readings are required to toggle touch state
        if (hysteresis > 2) {
            if (state == 0) {
                out_state = !out_state;
                digitalWrite(OUTPUT_PIN, out_state);
                state = 1;

                // Print when touch starts and the measured value
                // Can be commented out
                printf("START %d", timer);
                fflush(stdout); // Display instantly (by default only flushed on newline)
            }
            hysteresis = 0;
        } else if (hysteresis < -2) {
            if (state == 1) {
                state = 0;

                printf(" END\n");
            }
            hysteresis = 0;
        }
    }
    return 0;
}