serg10:
Hello, I am powering an Arduino Pro Mini 5v with a Lipo battery which has a maximum voltage of 4.2v and a minimum of 3.3v. I also need to power an NRF2401n+ with .9v - 3.6v but the voltage from a fully charged lipo is too high. I know I could lower the voltage with a diode, led or a resistor network but I was wondering which of these is most efficient?
Neither. Diodes and resistors don't work as power supplies when current draw is variable (eg. NRF2401n+).
Voltage drop across diodes is highly dependent on current. Resistors are just rubbish for power supplies.
A LM317 voltage regulator costs about 30 cents.