Apparently these nRF24L01+ modules need 3.3v, but I've only got 5v Trinket Pros that I'm using.
How can I reduce the voltage to the RF transmitter?
Would a voltage regulator work? Specifically looking at this: http://addicore.com/product-p/165.htm
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The NRF24L01+ has inputs that are 5V tolerant! So you only need to provide 3.3V on the Vcc pin of the module, which you can do by putting a voltage regulator between it, and the 5V of the Trinket. So there isn't really any need to convert the Trinket to 3.3V.
PS Remember to add some capacitors to the output of the voltage regulator, as the NRF needs a pretty stable 3.3V. Noise in the power lines can cause instability in the operation of the device.
I also had a 5V Pro Trinket on hand and needed to connect one of its 5V output pins to the 3V pairing pin on an Adafruit's EZ-Key HID. I solved it easily for the price of a couple 1/8 watt resistors by including a simple voltage divider in the circuit as a "level shifter" and it works like a charm. In my case it's not serial data but give it a try.
Classic voltage divider equation: V out = V in * R1 / (R1 + R2)
In setup(), set the 5V Pro Trinket digital out pin to be normally LOW when it's not doing anything in the sketch, e.g. if it were pin 8: pinMode(8,OUTPUT); digitalWrite(8, LOW);
Connect this 5V output pin to one end of a 1.8K resistor (the "R1").
Connect the other end of that resistor to both: your 3V module's input pin AND to a 3.3K resistor (the "R2") to ground.
If you have a meter, perhaps before connecting to your module input first double check your circuit at the juncture of the resistors. Actual serial data probably won't produce a reliable reading so temporarily alter your sketch to just take the 5V Pro Trinket pin HIGH. I measured 3.1 V output on mine when the output goes HIGH and it works great with the EZ-Key module.
As said by JayEye: there are 3,3V Trinkets and that is probably the cheapest option.
If you want to pay the extra money, there are special boards for it by addicore: https://www.amazon.com/gp/product/B00VFNZN8K/ref=s9_dcacsd_bhz_bw_c_x_3
third option is to build yourself a small voltage regulating circuit. The regulator that you linked to will do.