Radio doorbell on an Arduino Uno

// Radio doorbell, the sending end.
//
// Press the button and this Uno sends a packet over its nRF24L01 to the other
// Uno, which rings. radio.write() waits for the other radio's acknowledgement
// and says whether it came: take the receiving board's program away, or put
// its radio on another channel, and every press prints "no answer".
//
// Both radios have to agree on three things: the channel (76 is RF24's
// default), the data rate, and the address, "00001" here. Change any one of
// them on one side only and the packets go nowhere.

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

RF24 radio(7, 8); // CE on 7, CSN on 8
const byte address[6] = "00001";

const int BUTTON = 2;
int presses = 0;

void setup()
{
    Serial.begin(9600);
    pinMode(BUTTON, INPUT_PULLUP);
    radio.begin();
    radio.openWritingPipe(address);
    radio.setPALevel(RF24_PA_MIN);
    radio.stopListening();
    Serial.println("Press the button to ring.");
}

void loop()
{
    if (digitalRead(BUTTON) == HIGH) {
        return;
    }
    presses++;
    char message[32] = "ding dong ";
    message[10] = (char)('0' + presses % 10);
    bool heard = radio.write(&message, sizeof(message));
    Serial.print(message);
    Serial.println(heard ? ": delivered" : ": no answer");
    // Wait for the button to come back up, so one press is one ring.
    while (digitalRead(BUTTON) == LOW) {
        delay(10);
    }
    delay(50);
}

// Radio doorbell, the receiving end.
//
// This Uno listens on the same channel and address as the sender. Each packet
// that arrives is printed and flashes the LED. The radio acknowledges every
// packet on its own, in hardware, before the sketch has even looked at it:
// that acknowledgement is what makes the sender's radio.write() return true.

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

RF24 radio(7, 8); // CE on 7, CSN on 8
const byte address[6] = "00001";

const int LED = 5;

void setup()
{
    Serial.begin(9600);
    pinMode(LED, OUTPUT);
    radio.begin();
    radio.openReadingPipe(0, address);
    radio.setPALevel(RF24_PA_MIN);
    radio.startListening();
    Serial.println("Listening.");
}

void loop()
{
    if (!radio.available()) {
        return;
    }
    char message[32] = "";
    radio.read(&message, sizeof(message));
    Serial.print("heard: ");
    Serial.println(message);
    digitalWrite(LED, HIGH);
    delay(300);
    digitalWrite(LED, LOW);
}