Im making a thermal alarm (dht22), which notify by sms (gsm sim800L) the temperature it's high. It has a lcd display wich shows the temperature and humidity, a buzzer and leds that also notify if the temperature is high. All connected to an Arduino Uno and controlling the time with a rtc ds1302.
My problem is that idk how to choose the power supply for the project. The idea is that the alarm will be connected always to the current, but in case the power failure, it has to keep on. Do you recommend the battery as the power supply all the time or it's better the battery and other power supply? Another question is that the gsm uses 3.7-4.4v, must i use another power supply for that board or with the biggest power supply and rectifing the voltage is enough? (And the same question in case i add boards that has different voltages)
I hope the questions are understable bc English isn't my mother tongue
Two comments, your power supply has to provide 2A peaks for the modem, and there’s no need for a dedicated RTC chip, because the cellular network will provide far better localised time reference than the 1307, and never needs to be set.
Read up on the AT+CCLK, command.
Setting up a battery with a float charge would keep the battery charged and when power fails keep the unit operating.