Generate 5VDC signal when external sensor input changes from high to low I am new to arduino.  I have a project in which I need an arduino to generate a +5VDC  signal in response to a change of status, (high/5VDC To low/ground) from an external source.  

The external source is a stand alone receiver/transmitter set which outputs +5VDC when the transmitter is out of range and ground when the transmitter is within range.

The goal is to have the arduino output +5VDC (triggering a series of events) only when the TX is within range of the RX (signal voltage is GRND vs. +5vDC).  The series of events are controlled via a stand alone event controller which is initiated by a +5VDC signal courtesy of the arduino.

I already have a RX/TX set which outputs +5VDC when the the TX is absent and GROUND when the TX is within range.  I also have the event controller.  I only need the arduino solution. 

I have an arduino UNO but I am open to other option if needed.

Solution: give me +5VDC when signal voltage (from receiver) is low (ground).

If anyone could point me in the right direction it would be greatly appreciated.  This is my first arduino project so I have much to learn.

Best regards
This is literally the simplest Arduino program possible.  I can't believe you couldn't find an example of making one pin HIGH when another goes LOW.  

void setup(){
  pinMode(yourInputPin, INPUT_PULLUP);
  pinMode(yourOutputPin, OUTPUT);
}

void loop(){
  if(digitalRead(yourInputPin) == LOW){
     digitalWrite(youOutputPin, HIGH);
  }
  else {
     digitalWrite(yourOutputPin, LOW);
  }
}


All you need to do is supply the appropriate pin numbers.