// pulse an output if switch changes to low
// https://forum.arduino.cc/t/switching-circuit-using-relays-and-ups-with-arduino/1019626/

//  Different color LEDs are used just to show the current relay status.
//  Suitable  transistors would be used for driving the relay in place of LEDs later .


constexpr uint8_t inputPin {7};  // input 5v from mobile charger for AC presence sensing.
constexpr uint8_t UPSrelay {9};  // UPS output 220v relay for appliances
constexpr uint8_t ONOFFsw {11};  // connected to ups power switch via a relay. (To simulate UPS ON/OFF)
constexpr uint8_t MAINSrelay {10};  //  MAINS output relay 220v for appliances


constexpr uint16_t intervalOn {1};
constexpr uint16_t intervalOff {3};

byte previousInputState = HIGH;

void pulseUpdate()
{
  uint8_t inputState = digitalRead(inputPin);
  if (inputState == LOW && previousInputState == HIGH)
  {
    Serial.println(F("AC Power OFF"));  // inputpin got LOW from 5v charger in case of AC failed.
    digitalWrite(MAINSrelay, LOW);  //  NAINS relay turned off
    delay(intervalOff); // simple 3 sec. delay
    digitalWrite(ONOFFsw, HIGH); // Power switch of UPS pressed (through a relay) to turn ON the UPS
    delay(intervalOn); // Keep pressing power button for 1 sec.
    digitalWrite(ONOFFsw, LOW);  // Power switch released . Now UPS is turned ON
    delay(intervalOff); // simple delay of 3sec. to let the UPS initialize after power ON
    digitalWrite(UPSrelay, HIGH); //  UPS relay turned ON , now appliances are running on battery power.
    previousInputState = inputState;
  }
  else if (inputState == HIGH && previousInputState == LOW)
  {
    Serial.println(F("AC Power ON")); // inputpin got high from 5v charger in case of AC restored.
    delay(intervalOff); // a simple 3 sec. delay
    digitalWrite(UPSrelay, LOW); // UPS relay turned OFF
    digitalWrite(ONOFFsw, HIGH); // Power switch of UPS pressed (through a relay) to turn OFF the UPS
    delay(intervalOff); // Keep pressing power button for 3 sec. to turn OFF.
    digitalWrite(ONOFFsw, LOW); // Power button released . Now UPS is turned OFF
    digitalWrite(MAINSrelay, HIGH); // Mains relay turned ON, now appliances are running on Mains power.
    previousInputState = inputState;
  }
}



void setup() {
  Serial.begin(115200);
  Serial.println(F("Switch an output based on another pin"));
  pinMode(ONOFFsw, OUTPUT);
  pinMode(UPSrelay, OUTPUT);
  delay(50);
}

void loop() {
  pulseUpdate();

}