How can I make One UNO R3 send a signal to another UNO R3 ??

Is there a way I can output per say pin 5 on one UNO to another UNO .. and if on the second UNO a digital signal is detected .. it will do something.

Example: UNO1 and UNO2

If the first UNO1's pin 5 goes HIGH - UNO2 will set it's pin 6 HIGH

If there is a GND between the two UNOs, you can connect an output pin on the first UNO to an input pin on the second UNO.
The 2nd UNO reads its input pin and the code does its work. . .

What is this for?

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Yes, very easily. connect a button to arduino on say pin 4. read the button's state, and then if the button is pressed, make pin 5 'HIGH'. then, on the other end, connect an led to pin 4. then, have arduino 2 read pin 5, and if it is HIGH, then make pin 4, the led, HIGH as well. (make sure you connect GND between both arduinos)

Why do you ask? Is there something bigger you had in mind? Because obviously this task can be done with one arduino.

edit: larry beat me to it :)

13, you must be at least 26 ;)

LarryD: 13, you must be at least 26 ;)

I wish :D

As a beginner, it's a good idea to place a resistor between the two Arduinos, so that if you accidentally make the connected pins on both Arduinos outputs, you won't do any damage.

1K would do the job nicely. Then, if you make a mistake, no more than 5mA would flow. (270 ohms is the lowest resistance you should really use, which would allow 19mA to flow.) An Arduino pin has an absolute maximum rating of 40mA per pin.