---

• PA3 reads the footswitch.
• PA1 controls relay K2 through Q2.
• PA2 controls the LED.
• PA6 controls U4 mute / anti-pop photorelay.
• One footswitch click toggles the pedal ON/OFF.
• In distortion mode: relay ON, LED ON.
• In bypass mode: relay OFF, LED OFF.
• PA6 turns mute ON only briefly during relay switching, then returns LOW so it does not mute EFFECT_OUT.

---

Text
PA3 / PIN_PA3 = footswitch input
PA1 / PIN_PA1 = relay control output
PA2 / PIN_PA2 = LED indicator
PA6 / PIN_PA6 = anti-pop mute / photorelay control

Text
Relay PA1:
LOW  = bypass / relay OFF
HIGH = distortion / relay ON

LED PA2:
HIGH = LED OFF
LOW  = LED ON

Mute PA6:
LOW  = mute OFF / normal audio
HIGH = mute ON / EFFECT_OUT muted

---

Text
Effect OFF / bypass
PA1 = LOW
LED OFF
PA6 = LOW

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Effect ON / distortion
PA1 = HIGH
Relay ON
LED ON
PA6 briefly HIGH during switching, then LOW

Text
Effect OFF / bypass
PA1 = LOW
Relay OFF
LED OFF
PA6 briefly HIGH during switching, then LOW

---

1. Install Arduino IDE 2.x.
   

   image
2. Install megaTinyCore.
3. Prepare Arduino Uno or Nano as a jtag2updi programmer.

---

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File → Preferences

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Arduino IDE → Settings

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Additional Boards Manager URLs

add:

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http://drazzy.com/package_drazzy.com_index.json

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Tools → Board → Boards Manager

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megaTinyCore

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megaTinyCore by Spence Konde

---

1. Download or open the jtag2updi project.
2. In Arduino IDE, select the programmer board.

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Tools → Board → Arduino AVR Boards → Arduino Uno

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Tools → Board → Arduino AVR Boards → Arduino Nano
Tools → Processor → ATmega328P or ATmega328P Old Bootloader

3. Select the correct port:

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Tools → Port

4. Open jtag2updi.ino.
5. Press the normal Upload button.
6. Wait for:

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Done uploading

---

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Arduino D6  → 4.7 kΩ → PA0 / UPDI ATtiny212
Arduino GND → target board GND
Arduino 5V  → target board +5V

For the first programming test, it is better not to connect the pedal 9V supply. Power the target board from the Arduino programmer 5V.

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10 µF between RESET and GND on Arduino
capacitor + → RESET
capacitor - → GND

---

Select:

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Tools → Board → megaTinyCore → ATtiny412/402/212/202
Tools → Chip → ATtiny212
Tools → Clock → 16 MHz internal
Tools → Programmer → jtag2updi
Tools → Port → Arduino programmer port

Do not use the normal Upload button.

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Sketch → Upload Using Programmer

On Windows:

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Ctrl + Shift + U

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Command + Shift + U

---

Text
distortion_relay_bypass_attiny212.ino

Cpp
/*
  Final Distortion Pedal Relay Bypass Firmware
  MCU: ATtiny212
  Core: Arduino IDE + megaTinyCore

  Footswitch type:
  - Latching / maintained footswitch
  - One physical click toggles effect ON/OFF once

  Pin mapping:
  PA3 = footswitch input
  PA1 = relay driver output, active HIGH
  PA2 = LED indicator, active LOW
  PA6 = anti-pop mute / photorelay control

  Logic:
  Effect OFF / bypass:
    PA1 LOW
    LED OFF
    PA6 LOW

  Effect ON / distortion:
    PA1 HIGH
    LED ON
    PA6 LOW

  PA6 HIGH is used only briefly during relay switching for anti-pop mute.
*/

#include <Arduino.h>

// Pins
constexpr uint8_t FOOTSWITCH_PIN = PIN_PA3;
constexpr uint8_t RELAY_PIN      = PIN_PA1;
constexpr uint8_t LED_PIN        = PIN_PA2;
constexpr uint8_t MUTE_PIN       = PIN_PA6;

// Relay logic
constexpr uint8_t RELAY_ON  = HIGH;
constexpr uint8_t RELAY_OFF = LOW;

// LED is active LOW
constexpr uint8_t LED_ON  = LOW;
constexpr uint8_t LED_OFF = HIGH;

// U4 photorelay mute logic
// PA6 HIGH = mute ON
// PA6 LOW  = mute OFF / normal audio
constexpr uint8_t MUTE_ON  = HIGH;
constexpr uint8_t MUTE_OFF = LOW;

// Timing
constexpr unsigned long DEBOUNCE_MS = 50;
constexpr unsigned long LOCKOUT_MS  = 300;

// Anti-pop mute timing
constexpr unsigned long MUTE_BEFORE_RELAY_MS = 30;
constexpr unsigned long RELAY_SETTLE_MS      = 70;

// State
bool effectOn = false;

uint8_t lastRawSwitchState = HIGH;
uint8_t stableSwitchState  = HIGH;

unsigned long lastRawChangeTime = 0;
unsigned long lastToggleTime    = 0;

void applyStableState()
{
  if (effectOn) {
    digitalWrite(RELAY_PIN, RELAY_ON);
    digitalWrite(LED_PIN, LED_ON);
  } else {
    digitalWrite(RELAY_PIN, RELAY_OFF);
    digitalWrite(LED_PIN, LED_OFF);
  }

  // Important: mute must be OFF during normal audio operation
  digitalWrite(MUTE_PIN, MUTE_OFF);
}

void switchEffectWithMute()
{
  // Mute audio output briefly before relay switching
  digitalWrite(MUTE_PIN, MUTE_ON);
  delay(MUTE_BEFORE_RELAY_MS);

  // Toggle effect state
  effectOn = !effectOn;

  // Switch relay and LED
  if (effectOn) {
    digitalWrite(RELAY_PIN, RELAY_ON);
    digitalWrite(LED_PIN, LED_ON);
  } else {
    digitalWrite(RELAY_PIN, RELAY_OFF);
    digitalWrite(LED_PIN, LED_OFF);
  }

  // Wait for relay contacts to settle
  delay(RELAY_SETTLE_MS);

  // Release mute for normal audio operation
  digitalWrite(MUTE_PIN, MUTE_OFF);
}

void setup()
{
  // Configure outputs first
  pinMode(RELAY_PIN, OUTPUT);
  pinMode(LED_PIN, OUTPUT);
  pinMode(MUTE_PIN, OUTPUT);

  // Safe startup state
  effectOn = false;
  digitalWrite(MUTE_PIN, MUTE_OFF);
  applyStableState();

  // Footswitch input
  pinMode(FOOTSWITCH_PIN, INPUT_PULLUP);

  // Initialize switch state
  lastRawSwitchState = digitalRead(FOOTSWITCH_PIN);
  stableSwitchState  = lastRawSwitchState;

  lastRawChangeTime = millis();
  lastToggleTime    = millis();
}

void loop()
{
  unsigned long now = millis();
  uint8_t rawSwitchState = digitalRead(FOOTSWITCH_PIN);

  // Raw state changed: restart debounce timer
  if (rawSwitchState != lastRawSwitchState) {
    lastRawSwitchState = rawSwitchState;
    lastRawChangeTime = now;
  }

  // Accept only stable state after debounce time
  if ((now - lastRawChangeTime) >= DEBOUNCE_MS) {

    // Latching footswitch:
    // toggle on any stable state change, not only LOW press
    if (rawSwitchState != stableSwitchState) {

      // Extra lockout prevents bounce/double toggle
      if ((now - lastToggleTime) >= LOCKOUT_MS) {
        stableSwitchState = rawSwitchState;
        switchEffectWithMute();
        lastToggleTime = now;
      }
    }
  }
}

---

Text
Effect OFF:
PA1 = 0V
PA2 = HIGH / LED OFF
PA6 = 0V
Relay OFF / bypass

Effect ON:
PA1 = 5V
PA2 = LOW / LED ON
PA6 = 0V after short mute pulse
Relay ON / distortion

---

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D2 cathode / stripe side → +5V / relay pin 1
D2 anode / no-stripe side → RELAY_COIL_NEG / relay pin 12 / Q2 collector

---

Text
1 footswitch click → distortion ON, relay ON, LED ON
next footswitch click → bypass ON, relay OFF, LED OFF

---

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D2 stripe/cathode side → +5V
D2 anode side → RELAY_COIL_NEG / Q2 collector
PA6 LOW during normal audio
PA6 HIGH only briefly during switching
PA1 controls relay through Q2
PA2 LED logic is active LOW

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Bypass:
PA1 LOW
LED OFF
PA6 LOW

Distortion:
PA1 HIGH
LED ON
PA6 LOW after short mute pulse