The Seven-Segment LED Display

// Four-digit seven-segment multiplex driver, plus a flicker-fusion experiment.
//
// Segments a..g share seven pins (each through 220R). Each digit's common cathode is
// switched to ground by an NPN (base through 1k). Scan: drive the segment pattern for
// one digit, enable that digit, wait, blank, next digit. Fast enough and the eye fuses
// four flashing digits into one steady number.
//
// The experiment: sweep the refresh rate DOWN from 200 Hz and read the serial output to
// find the exact rate at which the number starts to flicker for YOUR eyes.

const int SEG[7] = {13, 12, 14, 27, 26, 25, 33};   // a b c d e f g
const int DIG[4] = {32, 4, 16, 17};                 // digit-enable (NPN bases)

// Segment patterns for 0-9, bit order a..g (1 = lit). Common cathode: HIGH lights a segment.
const uint8_t FONT[10] = {
  0b1111110, // 0  a b c d e f
  0b0110000, // 1  b c
  0b1101101, // 2  a b d e g
  0b1111001, // 3  a b c d g
  0b0110011, // 4  b c f g
  0b1011011, // 5  a c d f g
  0b1011111, // 6  a c d e f g
  0b1110000, // 7  a b c
  0b1111111, // 8  all
  0b1111011, // 9  a b c d f g
};

int digitValue[4] = {1, 2, 3, 4};

void blank() {
  for (int d = 0; d < 4; d++) digitalWrite(DIG[d], LOW);   // all digits off
}

// Show one digit's pattern for onMicros, then blank. Common cathode: segment HIGH = on,
// digit-enable HIGH drives the NPN and pulls that common to ground.
void showDigit(int pos, int val, unsigned long onMicros) {
  blank();
  uint8_t pat = FONT[val % 10];
  for (int s = 0; s < 7; s++) digitalWrite(SEG[s], (pat >> (6 - s)) & 1);
  digitalWrite(DIG[pos], HIGH);
  delayMicroseconds(onMicros);
  blank();
}

// One full frame = all four digits scanned once, at a given whole-display frame rate.
void frame(float frameHz) {
  unsigned long perDigit = (unsigned long)(1e6 / (frameHz * 4));
  for (int d = 0; d < 4; d++) showDigit(d, digitValue[d], perDigit);
}

void setup() {
  Serial.begin(115200);
  delay(300);
  for (int s = 0; s < 7; s++) { pinMode(SEG[s], OUTPUT); digitalWrite(SEG[s], LOW); }
  for (int d = 0; d < 4; d++) { pinMode(DIG[d], OUTPUT); digitalWrite(DIG[d], LOW); }

  Serial.println("# Seven-segment multiplex + flicker-fusion sweep");
  Serial.println("# Watch the display, straight on AND out of the corner of your eye.");
  Serial.println("# Note the rate printed when it FIRST starts to flicker. That is your");
  Serial.println("# flicker-fusion threshold. Peripheral vision fuses at a HIGHER rate,");
  Serial.println("# so the corner of your eye will flicker before the centre does.");
}

void loop() {
  // Sweep the whole-display refresh from 200 Hz down to 20 Hz over ~40 s, holding each
  // rate long enough to judge. Then jump back to 200 and repeat.
  for (float f = 200.0; f >= 20.0; f -= 5.0) {
    Serial.print("refresh = "); Serial.print(f, 0); Serial.println(" Hz");
    unsigned long hold = millis();
    while (millis() - hold < 1500) frame(f);   // hold this rate ~1.5 s
  }
  Serial.println("--- back to 200 Hz (rock steady) and repeating ---");
}