Выпаяв из ультразвукового дальномера HC-SR04, УЗ-излучатель и приёмник, можно собрать систему ультразвуковой коммуникация между двумя контроллерами Arduino.
Разумеется, для усиления принимаемого сигнала понадобится дополнительная обвязка в виде усилителя LM386 и компаратора LM393.

Код Arduino для передачи

/*
  Developed by:  Eduardo Zola - Zola Lab 2018 - www.zolalab.com.br - [email protected]
  Ultrasonic Data Communication
  Copyright (c) 2015 Eduardo Zola.  All right reserved.
  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.
  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.
*/


void setup()
{
    Serial.begin(115200);
    pinMode(3,OUTPUT);
}

void loop()
{
   send("Ultrasonic communication by Zola Lab\n");
   send("Hello World\n\n");
}

void send(String msg)
{
   byte ch;
   unsigned int pos = 0;
   unsigned int sz = msg.length();
   while(pos
Код Arduino для приёма
/*
  Developed by:  Eduardo Zola - Zola Lab 2018 - www.zolalab.com.br - [email protected]
  Ultrasonic Data Communication
  Copyright (c) 2015 Eduardo Zola.  All right reserved.
  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.
  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.
*/

int pos = 0;
unsigned char CH = 0;
unsigned int bits1 = 0;
boolean capture = false;

void setup()
{
   Serial.begin(115200);
   pinMode(5,INPUT_PULLUP);
}

void loop()
{
   if(digitalRead(5))
   {
      bits1 = 0;
      unsigned long deltaT = millis();
      while(millis()-deltaT <= 10) if(digitalRead(5)) bits1 ++;
      //Serial.println(bits1);
      if(capture)
      {
         boolean b = 0;
         if(bits1 > 290 && bits1 < 600) b = 0;
         if(bits1 > 20 && bits1 < 290) b = 1;
         if(b) bitSet(CH,7-pos); else bitClear(CH,7-pos);
         //Serial.print(b);
         pos++;
         if(pos == 8)
         {
            Serial.print((char)CH);
            pos = 0;
            capture = false;
         }
      }
      if(bits1 > 600)
      {
         capture = true;
         pos = 0;
      }
   }
}

Ссылки

Hack an ultrasonic HC-SR04 and make it talk each other
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Ультразвуковой датчик измерения расстояния HC-SR04
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