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I recommend to use Arduino Nano 3.3V power version on both sides: transmitter and receiver. Very little power consumption, direct interface with nRF24L01+ and can be directly powered by LiFePo4 battery
Excellent post! Thinking about making similar system. What about:1) Having transmitter in deep sleep mode, waking up by reed switch to send alert2) Powering transmitter by LiFePo4 3.2V battery on solar charge