I am looking at creating a weather station that will ideally sit outside for long periods of time (upwards of 6 months). I have seen tutorials on how to reduce the power consumption of the pro mini 3.3v and they recommend removing the LED and 3.3v regulator. I plan on running the pro mini off of 2 x AA batteries. The sensors I am planning on using are the BME280, DS18B20 (more accurate temperature sensor), and a NRF24 to transmit to another arduino sitting inside.
From what I understand is that these sensors will not run on less than 3 volts and I have been told that i2c needs a 3.3v volt supply. In my country it will not be cost effective for me to buy a 3.3v regulator with a low quiescent current. Because of this I was thinking instead of using 2 x AA batteries in series, I could use 3 or switch to an 18650. So what I would like to know is will these battery configurations work with my sensors without the voltage regulator or should I just sacrifice battery life by leaving the on board regulator installed in order to have working sensors.
Thank you for the helpful datasheets. I can see that the sensors will work with the voltage ranges. I now have a question about i2c. From what I have looked at online, i2c works on either 3.3v or 5v. If the supply voltage to both the MCU and i2c sensor drops below 3.3v, say 2.9v will it still work.
[EDIT thanks to @herbschwarz] Anything over 60% of VCC until VCC+0.5V is seen as a HIGH by the processor. For the sensor it is not specified in the datasheet, but given the fact it can work from 1.8V I would expect a similar parameter.
The 3.3V regulator found on the typical clone Pro Mini is not a quality regulator, and may have a large dropout voltage requirement. You may have to provide over 4V just to give it enough headroom to regulate properly. Do you already have the Pro Mini? If so, what are the markings on the regulator chip?
I believe if all the I2C devices are powered at the same voltage, there will not be a problem with those transmissions. I don't know of a 3.3V requirement for I2C.
That should work fine initially. The only issue is how long it will continue to work as the batteries discharge. But if the NRF module can deal with it and still transmit ok, then I suspect powering everything directly from the two AAs would work pretty well, and probably have longer battery life than the datasheets would suggest.
Of course you have to deal with the cold, which could have a big impact on alkalines. I wonder if two Ultimate Lithium AAs wouldn't be the best choice for cold weather. They are nominally 1.5V, but more like 1.75-1.8V no load, and as I remember work very well at low temps.
