RF24: two Arduinos talking over nRF24L01 radios

// 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);
}