esp-wemos-audioswitcher/firmware_esp/firmware_esp.ino

#include <Homie.h>
#include <ArduinoOTA.h>

#define ROOM_NUMBERS  5

// Serial Protocol:
// 9600 Baud
// 1 char: room number 1-5
// 2 char: command v (volume) / r (relais)
// relais command:
//   3 char: a or b
// volume command:
//   3-5 char: 0-100 volume


long lastTickMillis;
uint8_t volume[ROOM_NUMBERS], volumeLast[ROOM_NUMBERS];
String relais[ROOM_NUMBERS], relaisLast[ROOM_NUMBERS];

HomieNode volumeNode("volume", "volume");
HomieNode switchNode("switches", "switch");

bool nodeInputHandlerVolume(const HomieRange& range, const String& value) {
  
  int vol = value.toInt();
  if(range.index >= 1 && range.index <= ROOM_NUMBERS && vol >= 0 && vol <= 100) {
      volumeNode.setProperty("volume").setRange(range).send(value);
      volume[range.index-1] = vol;
      return true;
  }
  return false;
}

bool nodeInputHandlerRelais(const HomieRange& range, const String& value) {
        
  if(range.index >= 1 && range.index <= 5 && (value == "a" || value == "b")) { 
      switchNode.setProperty("switch").setRange(range).send(value);
      relais[range.index-1] = value;
      return true;
  }
  
  return false;
}

void setup() {
  Serial.begin(9600);

  Homie_setFirmware("audiocontroller", "1.0.0");
  Homie.disableLogging();

  volumeNode.advertiseRange("volume", 1, ROOM_NUMBERS).settable(nodeInputHandlerVolume);
  switchNode.advertiseRange("switch", 1, ROOM_NUMBERS).settable(nodeInputHandlerRelais);

  Homie.setup();

  ArduinoOTA.setHostname(Homie.getConfiguration().deviceId);
  ArduinoOTA.begin();

  
}

void loop() {
  Homie.loop();
  ArduinoOTA.handle();

  // wait for at least 200ms before sending new command because
  // the arduino mini has a 100ms serial timeout
  
    for(uint8_t i = 0; i < ROOM_NUMBERS; i++) {
      
      if(volume[i] != volumeLast[i]) {
        if(millis() - lastTickMillis >= 400) {
          Serial.print(i+1);
          Serial.print("v");
          Serial.print(volume[i]);
          volumeLast[i] = volume[i];
          lastTickMillis = millis();
        }
      }

      if(relais[i] != relaisLast[i]) {
        if(millis() - lastTickMillis >= 400) {
          Serial.print(i+1);
          Serial.print("r");
          Serial.print(relais[i]);
          relaisLast[i] = relais[i];
          lastTickMillis = millis();
        }
      }
    }
    

  
}