Bluetooth home automation with relays on an Arduino Uno

// Bluetooth home automation: an HC-05 and a four-relay board, switched from a
// phone app by single characters, with each change answered back.
//
//   HC-05        TXD -> pin 2 (SoftwareSerial RX)
//                RXD <- pin 3 through 1k, with 2k from RXD to GND
//                VCC 5V, GND
//   pins 4 to 7  the relay board's IN1 to IN4 (VCC 5V, GND)
//   each relay   COM to the 12 V adapter's +, NO to its lamp, the lamp's
//                other lead to the adapter's -
//   adapter -    also to GND, so the 12 V side shares the Uno's ground
//
// The switch-panel apps ("Arduino Bluetooth Control" and the like) send one
// character per button. This sketch takes:
//
//   1 2 3 4   toggle that relay
//   A         all on        a   all off
//   ?         say which are on
//
// In Mokxi the far end of the HC-05 is the module's own box in the serial
// monitor: type 1 there and press Enter, and relay 1 clicks over and lights
// its lamp. There is no radio here, only the UART a paired phone would talk
// to, so anything about pairing or range is left to the real module.
//
// The relay board is active low: LOW pulls a relay in. Each pin is written
// HIGH before it becomes an output so nothing clicks at power-up. The HC-05's
// RXD is a 3.3 V input, which is what the 1k and 2k divider is for.

#include <SoftwareSerial.h>

SoftwareSerial bluetooth(2, 3);  // RX, TX

const int RELAYS[4] = {4, 5, 6, 7};
bool on[4] = {false, false, false, false};

void report() {
  for (int i = 0; i < 4; i++) {
    bluetooth.print(on[i] ? '1' : '0');
    Serial.print(on[i] ? '1' : '0');
  }
  bluetooth.println();
  Serial.println();
}

void set(int i, bool state) {
  on[i] = state;
  digitalWrite(RELAYS[i], state ? LOW : HIGH);  // active low
  bluetooth.print("Relay ");
  bluetooth.print(i + 1);
  bluetooth.println(state ? " ON" : " OFF");
  Serial.print("relay ");
  Serial.print(i + 1);
  Serial.println(state ? " ON" : " OFF");
}

void setup() {
  for (int i = 0; i < 4; i++) {
    digitalWrite(RELAYS[i], HIGH);  // off before it is an output
    pinMode(RELAYS[i], OUTPUT);
  }
  Serial.begin(9600);
  bluetooth.begin(9600);  // the HC-05's factory baud
  Serial.println("Bluetooth relays: 1-4 toggle, A all on, a all off, ? status");
}

void loop() {
  if (!bluetooth.available()) return;
  char c = bluetooth.read();
  if (c >= '1' && c <= '4') {
    int i = c - '1';
    set(i, !on[i]);
  } else if (c == 'A' || c == 'a') {
    for (int i = 0; i < 4; i++) set(i, c == 'A');
  } else if (c == '?') {
    report();
  }
}