• MCU: U2 — Microchip ATTINY212-SSN
• Framework: Arduino IDE with megaTinyCore
• Clock: Internal oscillator; choose a normal megaTinyCore ATtiny212 board configuration for your hardware.
• Programmer: UPDI via the schematic's PA0/UPDI connection.
• Libraries: None beyond the Arduino core.

• Power-up default: bypass / effect OFF / relay OFF.
• Every valid footswitch press toggles the effect state once.
• Holding the footswitch does not repeatedly toggle.
• Debounce time: 40 ms, within the requested 30–50 ms range.
• Relay output follows effect state: effect ON energizes the relay; effect OFF de-energizes it.
• LED follows effect state using active-low drive based on the schematic.

1. Install megaTinyCore in Arduino IDE.
2. Select an ATtiny412/212-series board target and choose ATtiny212 as the chip.
3. Select the appropriate clock option for your board.
4. Upload using a UPDI programmer connected to PA0/UPDI, VCC, and GND.

Cpp
/*
  Distortion Effect Pedal Relay Bypass Firmware
  MCU: ATtiny212 (U2, ATTINY212-SSN)
  Core: Arduino IDE + megaTinyCore

  Schematic-derived connections:
    FOOT_SW_NODE       -> U2 PA3, physical pin 7, active LOW input with pull-up
    Relay driver input -> U2 PA1 through R4 to Q2 base
    RELAY_COIL_NEG     -> Q2 collector / relay coil negative, pulled LOW when Q2 is ON
    LED indicator      -> U2 PA2 to D1, with R2 to +5V; assumed active LOW

  Power-up state:
    Effect OFF / bypass / relay OFF / LED OFF
*/

#include <Arduino.h>

// ─── Pin Definitions — from schematic ─────────────────────────────
constexpr uint8_t FOOTSWITCH_PIN = PIN_PA3;   // U2 PA3, physical pin 7, FOOT_SW_NODE
constexpr uint8_t RELAY_DRIVE_PIN = PIN_PA1;  // U2 PA1, physical pin 4, drives Q2 base via R4
constexpr uint8_t LED_PIN = PIN_PA2;          // U2 PA2, physical pin 5, D1 indicator LED
constexpr uint8_t MUTE_PHOTORELAY_PIN = PIN_PA6; // U2 PA6, physical pin 2, U4 input via R3; unused here

// ─── Active Levels ────────────────────────────────────────────────
constexpr uint8_t FOOTSWITCH_PRESSED_LEVEL = LOW;  // switch assumed to pull FOOT_SW_NODE to GND
constexpr uint8_t RELAY_ON_LEVEL = HIGH;           // PA1 HIGH turns Q2 on and energizes relay
constexpr uint8_t RELAY_OFF_LEVEL = LOW;

// LED appears connected from +5V through R2/D1 into PA2, so PA2 sinks current when LOW.
constexpr uint8_t LED_ACTIVE_LEVEL = LOW;
constexpr uint8_t LED_INACTIVE_LEVEL = HIGH;

// Optional anti-pop photorelay is left off. Adjust if you later add mute timing.
constexpr uint8_t MUTE_INACTIVE_LEVEL = LOW;

// ─── Debounce Configuration ───────────────────────────────────────
constexpr unsigned long DEBOUNCE_TIME_MS = 40;  // requested 30–50 ms range

// ─── State Variables ──────────────────────────────────────────────
bool effectOn = false;

uint8_t lastRawFootswitchLevel = HIGH;
uint8_t debouncedFootswitchLevel = HIGH;
unsigned long lastDebounceChangeMs = 0;

// ─── Output Helpers ───────────────────────────────────────────────
void applyEffectState()
{
  if (effectOn) {
    // Effect ON: energize relay so audio path switches to distortion circuit.
    digitalWrite(RELAY_DRIVE_PIN, RELAY_ON_LEVEL);
    digitalWrite(LED_PIN, LED_ACTIVE_LEVEL);
  } else {
    // Effect OFF: relay off, bypass mode.
    digitalWrite(RELAY_DRIVE_PIN, RELAY_OFF_LEVEL);
    digitalWrite(LED_PIN, LED_INACTIVE_LEVEL);
  }
}

void setDefaultBypassState()
{
  effectOn = false;
  applyEffectState();
}

// ─── Setup ────────────────────────────────────────────────────────
void setup()
{
  // Configure outputs first and force safe defaults before enabling input logic.
  pinMode(RELAY_DRIVE_PIN, OUTPUT);
  pinMode(LED_PIN, OUTPUT);
  pinMode(MUTE_PHOTORELAY_PIN, OUTPUT);

  digitalWrite(MUTE_PHOTORELAY_PIN, MUTE_INACTIVE_LEVEL);
  setDefaultBypassState();

  // Footswitch input uses internal pull-up because the switch is assumed to pull to GND.
  pinMode(FOOTSWITCH_PIN, INPUT_PULLUP);

  // Initialize debounce state from the current input level.
  lastRawFootswitchLevel = digitalRead(FOOTSWITCH_PIN);
  debouncedFootswitchLevel = lastRawFootswitchLevel;
  lastDebounceChangeMs = millis();
}

// ─── Main Loop ────────────────────────────────────────────────────
void loop()
{
  const unsigned long nowMs = millis();
  const uint8_t rawLevel = digitalRead(FOOTSWITCH_PIN);

  // If the raw input changed, restart debounce timing.
  if (rawLevel != lastRawFootswitchLevel) {
    lastRawFootswitchLevel = rawLevel;
    lastDebounceChangeMs = nowMs;
  }

  // Accept a new stable input state after debounce time has elapsed.
  if ((nowMs - lastDebounceChangeMs) >= DEBOUNCE_TIME_MS &&
      rawLevel != debouncedFootswitchLevel) {

    debouncedFootswitchLevel = rawLevel;

    // Toggle only on the transition to the pressed state.
    // This prevents repeated toggles while the switch is held.
    if (debouncedFootswitchLevel == FOOTSWITCH_PRESSED_LEVEL) {
      effectOn = !effectOn;
      applyEffectState();
    }
  }
}

• The schematic shows FOOT_SW_NODE at U2 PA3, but only firmware-level behavior can confirm the external footswitch polarity. If the switch does not pull to ground, change pinMode(FOOTSWITCH_PIN, INPUT_PULLUP) and FOOTSWITCH_PRESSED_LEVEL accordingly.
• RELAY_COIL_NEG is not directly driven by U2; it is the relay coil low-side node at Q2 collector. U2 drives Q2 through PA1/R4, so the correct firmware output is PA1, active HIGH.
• The LED is inferred as active LOW because R2 goes to +5V and D1 then connects to PA2. If testing shows inverted LED behavior, swap LED_ACTIVE_LEVEL and LED_INACTIVE_LEVEL.
• PA6 appears to drive the TLP3431 anti-pop photorelay input through R3. The requested firmware does not sequence mute timing, so it is held inactive.