An alternative is to do pullup via an output pin. So connect the switch like this:
D2 --- 47k --- D1 --- reed switch --- GND
Disable D1's pullups. While sleeping set D2 low to eliminate all power draw, then after wakeup you can set D2 high, wait a few ms and then measure D1. Once done, set D2 low again.
You can't do interrupt based sensing this way but works for polling.
I've been doing an experiment based on my custom board (https://github.com/buxtronix/mys-pro-mini) which is essentially an Arduino-pro-mini compatible pinout with NRF radio and SHT31 temp/humidity sensor built in.
It can take a CR2032 directly on board.
I've been running two identical sensors, the only difference being that one uses the INTRC clock, and the other uses an 8MHz Xtal. Update rate is about 1/min.
As you can see, I'm nearly 6 months in and I think I should be able to get to a year. Interestingly, there is no notable difference in battery between the two clock sources - it's otherwise been widely believed that xtal consumes more power than intrc.
The only low-power prep I'm doing in the code is to disable the ADC and set all unused floating pins to output mode. Standard arduino bootloader, BOD disabled.
Confirming also, I have about a dozen nodes around the place, all running off 2xAA batteries and all of the DS18B20 devices have no problems with this supply voltage, even when they are down to 2.8V they're still reporting reliably.
@Boots33 I haven't done a writeup of mine, perhaps I should fish it out of the pool sometime and get some photos.
The DS18B20 has a tiny standby current, something like 750nA. Even with 30 second poll intervals, I'm finding my sensors typically get near-shelf life from the batteries.
The physical setup is pretty basic - just a Pro Mini, battery, radio, and ds18b20. There's no circuitry for regulators, dc convertors or battery sensing. Of course I did remove the onboard regulator as it will draw power from the 'out' pin.
One thing I had to do was reprogram the fuses - you may find the Pro Mini comes with the BoD fuses set to like 2.7v. You can set it down to 1.8v so you'll get much longer life out of it. The effort is worth avoiding the hassle of more frequent battery changes.
Battery level sensing uses this VCC library: https://github.com/Yveaux/Arduino_Vcc
Nice project! I did one also with the same original thermometer. A couple of differences with mine:
In the past 5 months my battery voltage has stayed at around 2.97v.
For those looking to 3d print a case for this, if you cant find the right sized ducting, I designed one thats unobtrusive:
All of the ducting near me is a bit too slim to fit sensors based on this, so I decided to design and print a case instead.
Does the trick nicely, and I also chose to use proper AA battery clips to retain a good connection, as the cable-tie battery assembly wasn't holding well for me!
On the Thingiverse page I have a link to the original OnShape doc.
https://www.thingiverse.com/thing:2418194
Thoughts, suggestions, etc will be much appreciated!