user65965: 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

user33871: 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.  

user4143: Why waste an Arduino on such a paltry job?  Here's a circuit that has just 3  components in it that does what you want:

simulate this circuit – Schematic created using CircuitLab
It's basically a NOT gate (you can use an actual NOT gate, like a 74HC04 if you like) where the output is the opposite of the input.  You could even remove R2 if you like - that just prevents the input floating when there's no valid signal detected.
This form of gate is called RTL - Resistor-Transistor Logic, and is one of the simplest logic gate arrangements there is.

65965: <p>I am new to arduino.  I have a project in which I need an arduino to generate a +5VDC  signal in response to a <em>change</em> <em>of</em> <em>status</em>, (high/5VDC To low/ground) from an external source.  </p>

<p>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.</p>

<p>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.</p>

<p>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. </p>

<p>I have an arduino UNO but I am open to other option if needed.</p>

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

<p>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.</p>

<p>Best regards</p>
 33871: <p>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.  </p>

<pre><code>void setup(){
  pinMode(yourInputPin, INPUT_PULLUP);
  pinMode(yourOutputPin, OUTPUT);
}

void loop(){
  if(digitalRead(yourInputPin) == LOW){
     digitalWrite(youOutputPin, HIGH);
  }
  else {
     digitalWrite(yourOutputPin, LOW);
  }
}
</code></pre>

<p>All you need to do is supply the appropriate pin numbers.  </p>
 4143: <p>Why waste an Arduino on such a paltry job?  Here's a circuit that has just 3  components in it that does what you want:</p>



<p><img src="https://i.stack.imgur.com/VUQRA.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fVUQRA.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>

<p>It's basically a NOT gate (you can use an actual NOT gate, like a 74HC04 if you like) where the output is the opposite of the input.  You could even remove R2 if you like - that just prevents the input floating when there's no valid signal detected.</p>

<p>This form of gate is called <em>RTL</em> - Resistor-Transistor Logic, and is one of the simplest logic gate arrangements there is.</p>
