Send a heartbeat on an Arduino Uno

// Send a heartbeat: a pair of these, one for you and one for someone you
// love. Press the button on yours and the heart on theirs beats twice, lub-dub,
// and yours glows once to say it got there.
//
//   radio     nRF24L01: CE -> pin 7, CSN -> pin 8, SCK -> 13, MOSI -> 11,
//             MISO -> 12, VCC -> 3V3 (never 5 V), GND -> GND
//   button    pin 2 -> button -> GND        (INPUT_PULLUP, pressed reads LOW)
//   heart     pin 5 -> 220 ohms -> red LED -> GND
//
// Both boards run this same sketch. Each one listens all the time, and only
// stops listening for the moment it sends. They share one address, so either
// can start, and the radio that hears the packet answers it in hardware:
// radio.write() returns true only when that answer comes back. That one bool
// is how the sender knows the heartbeat arrived.
//
// The radios reach across a house, not across a city. For two homes far
// apart the same idea runs over Wi-Fi on an ESP32, but the heartbeat and the
// lub-dub are the same.
//
// Things to try. Press the button on the left board, then on the right one.
// Delete one board and press again: the sender says nobody answered.

#include <SPI.h>
#include <nRF24L01.h>
#include <RF24.h>

const int BUTTON_PIN = 2;
const int HEART_PIN = 5;

RF24 radio(7, 8);   // CE, CSN
const byte address[6] = "HEART";

bool wasPressed = false;
int sent = 0;

void fade(int peak, int ms) {
  for (int v = 0; v <= peak; v += 15) {
    analogWrite(HEART_PIN, v);
    delay(ms / 34);
  }
  for (int v = peak; v >= 0; v -= 15) {
    analogWrite(HEART_PIN, v);
    delay(ms / 34);
  }
  digitalWrite(HEART_PIN, LOW);
}

void beat() {
  fade(255, 250);   // lub
  delay(80);
  fade(150, 200);   // dub
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(HEART_PIN, OUTPUT);
  radio.begin();
  radio.setPALevel(RF24_PA_LOW);
  radio.openWritingPipe(address);
  radio.openReadingPipe(1, address);
  radio.startListening();
  Serial.println("Heartbeat ready: press the button to send one");
}

void loop() {
  bool pressed = digitalRead(BUTTON_PIN) == LOW;
  if (pressed && !wasPressed) {
    sent++;
    char message[8] = "beat";
    radio.stopListening();
    bool heard = radio.write(&message, sizeof(message));
    radio.startListening();
    Serial.print("Heartbeat ");
    Serial.print(sent);
    if (heard) {
      Serial.println(" sent and received");
      fade(60, 300);   // a soft glow: it got there
    } else {
      Serial.println(": nobody answered");
    }
    delay(50);
  }
  wasPressed = pressed;

  if (radio.available()) {
    char message[8] = "";
    radio.read(&message, sizeof(message));
    Serial.println("A heartbeat for you");
    beat();
  }
}

// Send a heartbeat: a pair of these, one for you and one for someone you
// love. Press the button on yours and the heart on theirs beats twice, lub-dub,
// and yours glows once to say it got there.
//
//   radio     nRF24L01: CE -> pin 7, CSN -> pin 8, SCK -> 13, MOSI -> 11,
//             MISO -> 12, VCC -> 3V3 (never 5 V), GND -> GND
//   button    pin 2 -> button -> GND        (INPUT_PULLUP, pressed reads LOW)
//   heart     pin 5 -> 220 ohms -> red LED -> GND
//
// Both boards run this same sketch. Each one listens all the time, and only
// stops listening for the moment it sends. They share one address, so either
// can start, and the radio that hears the packet answers it in hardware:
// radio.write() returns true only when that answer comes back. That one bool
// is how the sender knows the heartbeat arrived.
//
// The radios reach across a house, not across a city. For two homes far
// apart the same idea runs over Wi-Fi on an ESP32, but the heartbeat and the
// lub-dub are the same.
//
// Things to try. Press the button on the left board, then on the right one.
// Delete one board and press again: the sender says nobody answered.

#include <SPI.h>
#include <nRF24L01.h>
#include <RF24.h>

const int BUTTON_PIN = 2;
const int HEART_PIN = 5;

RF24 radio(7, 8);   // CE, CSN
const byte address[6] = "HEART";

bool wasPressed = false;
int sent = 0;

void fade(int peak, int ms) {
  for (int v = 0; v <= peak; v += 15) {
    analogWrite(HEART_PIN, v);
    delay(ms / 34);
  }
  for (int v = peak; v >= 0; v -= 15) {
    analogWrite(HEART_PIN, v);
    delay(ms / 34);
  }
  digitalWrite(HEART_PIN, LOW);
}

void beat() {
  fade(255, 250);   // lub
  delay(80);
  fade(150, 200);   // dub
}

void setup() {
  Serial.begin(9600);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(HEART_PIN, OUTPUT);
  radio.begin();
  radio.setPALevel(RF24_PA_LOW);
  radio.openWritingPipe(address);
  radio.openReadingPipe(1, address);
  radio.startListening();
  Serial.println("Heartbeat ready: press the button to send one");
}

void loop() {
  bool pressed = digitalRead(BUTTON_PIN) == LOW;
  if (pressed && !wasPressed) {
    sent++;
    char message[8] = "beat";
    radio.stopListening();
    bool heard = radio.write(&message, sizeof(message));
    radio.startListening();
    Serial.print("Heartbeat ");
    Serial.print(sent);
    if (heard) {
      Serial.println(" sent and received");
      fade(60, 300);   // a soft glow: it got there
    } else {
      Serial.println(": nobody answered");
    }
    delay(50);
  }
  wasPressed = pressed;

  if (radio.available()) {
    char message[8] = "";
    radio.read(&message, sizeof(message));
    Serial.println("A heartbeat for you");
    beat();
  }
}