user3540: I have seen several references where the solution to interfacing a 5V Arduino with a 3.3V nRF24L01+ was to power the nRF24L01+ externally.
How do I do this?  The ground of the nRF24L01+ is connected to the Arduino; can I simply attach 3.3V power between the power and ground pins of the nRF24L01+?

user3521: Yes, You can.
In any communication system, each module should have the same reference point of operation, or else the line is said to float. Here it is GND of arduino mini and GND of nRF24L01+.

user337: To connect the nRF24L01+ to a 5V Pro Mini you will need external 3.3V power supply to power the nRF24L01+.  The ground for the Pro Mini and the ground for the nRF24L01+ should be tied together to ensure a common ground reference.
The inputs to the nRF24L01+ are 5V tolerant so you will not need a level shifter to convert the 5V Digital IO on the Pro Mini to 3.3V. Be aware the not all 3.3V devices have 5V tolerant inputs like nRF24L01+.
You might want to look into using a 3.3V Pro Mini with that you should be able to use the regulated 3.3V power on the Pro Mini to power the nRF24L01+. Be aware that the 3.3V Pro Mini only runs at 8MHz instead of 16MHz.

3540: <p>I have seen several references where the solution to interfacing a 5V Arduino with a 3.3V nRF24L01+ was to power the nRF24L01+ externally.</p>

<p>How do I do this?  The ground of the nRF24L01+ is connected to the Arduino; can I simply attach 3.3V power between the power and ground pins of the nRF24L01+?</p>
 3521: <p>Yes, You can.
In any communication system, each module should have the same reference point of operation, or else the line is said to float. Here it is GND of arduino mini and GND of nRF24L01+.</p>
 337: <p>To connect the nRF24L01+ to a 5V Pro Mini you will need external 3.3V power supply to power the nRF24L01+.  The ground for the Pro Mini and the ground for the nRF24L01+ should be tied together to ensure a common ground reference.</p>

<p>The inputs to the nRF24L01+ are 5V tolerant so you will not need a level shifter to convert the 5V Digital IO on the Pro Mini to 3.3V. Be aware the not all 3.3V devices have 5V tolerant inputs like nRF24L01+.</p>

<p>You might want to look into using a <a href="https://www.sparkfun.com/products/11114" rel="nofollow">3.3V Pro Mini</a> with that you should be able to use the regulated 3.3V power on the Pro Mini to power the nRF24L01+. Be aware that the 3.3V Pro Mini only runs at 8MHz instead of 16MHz.</p>
