Background: I am creating climate sensors for a master's project. My Arduino board is a Nano Every. I have the AHT20, LTR390, an anemometer outputting a voltage to an analog pin, and a DS3231 RTC. I also have an SD card module for storing the data, which I am running at 3.3V. The measurements and time are recording once a minute then the arduino goes into SLEEP_MODE_PWR_DOWN until the next minute. I will only be visiting the sensors in the field once a month, so I need the batteries to last that long, and I am also hoping to keep the project as low cost as possible.
My question is, is it safe to power the project through 4.5V to the 5V pin, and how long will this last? My original plan was to connect a battery holder with 4 D-cell batteries to the VIN pin, but I have since realised that the 6V are not sufficient to power the circuit. Everything runs smoothly when I have three D-cells sending 4.5V through the 5V pin, but I intend to leave the sensors for a month before changing the batteries, and I don't know if the low voltage will sustain the project as the batteries' power drains.
Disclaimer, I am an entomologist, not a programmer and have only learnt hardware and software over the last two months, so I am a novice.
I'm not sure I have this entirely correct, but I think the current will be as follows.
Powered on: Every (25mA) + AHT20 (980microA) + LTR390 (100-110 microA) + DS3231 (500 nA) + SD card (100mA) = just over 126mA
Deep sleep: Every(10mA) + AHT20 (250nA) + LTR390 (1 microA) + DS3231 (500nA) + SD card (0.2mA) = just over 10mA
I couldn't really find information on the power consumption of the Nano or the SD card module, both of those didn't have the information in their data sheets. I've just assumed 25/10mA for the Nano based on other forum posts and 100mA for the SD card when reading to be safe.
Entomology is a science, so I would imagine some familiarity with maths is required...
I think you should measure the current consumed, on the test bench. Also the time spent in sleep and wake modes. Then you can calculate an overall average consumption.
Also knowing your battery capacity will allow you to calculate an approximate battery life. But, again, testing on the bench is going to be more accurate. Unfortunately the test might take a long time!
The Every has a schottky diode between VUSB and the 5V pin. So the computer would be protected, but the batteries might be at risk.
But what about four cells in series connected to the 5V pin? The AP2112 3.3V regulator on the Every can take up to 6.5V input, and can supply 600mA. Then you would have to run all the sensors and SD card at 3.3V since you wouldn't have a regulated 5V anymore. But that would give you maximum battery life.
The big current consumer here is probably the SD card. My understanding is that different brands vary a good bit in how much current they draw when idle. So some research on that might be useful, or you could test the ones you're using. One alternative would be to collect readings in the Every, and only write them to the SD card once a day or whatever, and power down the SD module entirely in between. You would need a mosfet for that. I'd like to know more about the SD module you're using. Perhaps a photo of it.
Not true.
If the USB was at 5.25V and the batteries are at 4.5V or less the diode would conduct with nothing to limit the current and blow out the USB port.
I don't know if alkaline batteries sink much current during an attempted "charge". But then all the more reason to use four cells to the 5V pin, and you could even give them their own schottky. Then both are protected.
My apologies. I was thinking the Every ran on 3.3V, but it appears it is 5V. So four cells wouldn't work without some other regulator/converter thrown in to produce 5V.
I will never have the USB and batteries connected at the same time, once the code is uploaded the sensors will only be battery powered in the field. If I do need to connect the USB for any reason, I can remove the batteries first.
My SD card module has a 3.3v and 5v pin on it. I have attached a photo of the model. It seems to work just fine on the Every's 3.3v pin and my data has been logging to the card. Will there be issues with this long term? I can switch to the 5v pin.
