If you have a single button which is to switch on an NRF24L01 for a few seconds then the whole thing powers down, another option is to use a latch circuit What is correct way to make latch power ciruit for arduino? - #9 by ShermanP. The button switches the latch on and everything powers up and the transmitter sends its payload. After a predefined time, the program breaks the latch and it all powers down. I'll still be curious to see how far you get using the watch dog timer in this role, however.