I present here the part of the circuit I am using to power up/down the Arduino every 15 minutes. This circuit is adapted from Rui Santos’ original work that you can find here: https://randomnerdtutorials.com/power-saving-latching-circuit/. 

The RTC-DS3231 module (J1 on the figure) and the Aduino MEGA PRO (J2) are both powered by a 6V battery (with a solar panel) represented here by the connector SV1. Since the working voltage of the RTC module is around 3.3V, I am using a stepdown module (SV2). 

Only the RTC module is continuously powered. Every 15 minutes, the RTC alarm (pin SQW of J1) is triggered, switching in serie the NPN transistor (2N3904) and the P-MOSFET (Q1), thus powering up the Arduino. 

Once powered, the Arduino takes over the SQW signal by setting HIGH D3, thus keeping the circuit closed. 

After all operation are done, the new alarm is set and the Arduino is power down by setting D3 LOW, which will open again the circuit. 

The schematic and printed circuit board can be found in EAGLE and PDF format on the project git. This board is a prototype and is not optimized! It is designed to be easily reproduced by hand.