If anyone more knowledgeable would like to share their insights or favourite ressources on the matter I'd be really interested.
For low power you can obv turn off radio when youre not using it. Probably some simple way to sleep/shut it off completely as well.
You can communicate over the 802.15.4 radio which is very low power compared to WiFi.
I'd use RIOT-OS, Zephyr has official support from Nordic, but that's what I'm familiar with: You can create a 6loWPAN network where each node gets an IPv6 address, they can communicate to the IPv6 world through a border router which would be another nRF52 Dongle where the USB serves as an USB-Ethernet Uplink that you plug into your router.
You could then use CoAP to send the data to a server somewhere (or just plain UDP) and sleep most of the time.
https://en.wikipedia.org/wiki/Comparison_of_802.15.4_radio_m...
You can use SPI Radios though (if you don't want to design a board yourself). CC1101 isn't IEEE 802.15.4 but if you don't want to talk to third party modules that shouldn't matter.
AT86RF215 is a dual-band chip with pretty good range, but besides the official (expensive) eval board, there is no ready-to-buy board. You could manufacture them yourself though. [0]
Of course there is also LoRa which would likely be enough for your use case. It just has a much lower frame size.
I made a small MPPT solar harvester with a 0.47F supercap. This system reports soil moisture without using battery. It works even when weather is cloudy.
Hope this gives you some insight.
To receive, you can buy corresponding modules but I wouldn't bother with that. Get an RTL-SDR dongle for whatever computer you want to process the data on (an RPi is great for this if you don't want to keep your desktop on all the time to do it), and use the program rtl_433.
The 433 MHz transmitter module simply sends a 433ish MHz continuous signal when its data input pin is high and does not send when the input pin is low. You encode your message into a series of pulses of the transmitter.
For example, for a project I'm doing using one of these transmitter modules I've currently got it on an Arduino for testing. Here's the Arduino code to send a message.
#define CYCLE 1000
#define W_0 600
#define W_1 400
#define INTRO_1 1500
#define INTRO_0 1500
void send_message(char * bp, int n)
{
digitalWrite(RADIO, HIGH);
delayMicroseconds(INTRO_1);
digitalWrite(RADIO, LOW);
delayMicroseconds(INTRO_0);
while (n-- > 0) {
int b = *bp++ & 0xff;
int m = 0x80;
while (m != 0) {
int w = (b & m) ? W_1 : W_0;
digitalWrite(RADIO, HIGH);
delayMicroseconds(w);
digitalWrite(RADIO, LOW);
delayMicroseconds(CYCLE-w);
m >>= 1;
}
}
}
My current test messages all start with "TZS", are 12 bytes long, and the last 2 bytes are a counter that simply increments every time I press a button. Adding this to an rtl_433 config file makes rtl_433 recognize those messages and print the count:
decoder {
name = TEST,
modulation = OOK_PWM,
short = 424,
long = 624,
reset = 1500,
sync = 1528,
bits = 96,
match = {24}0x849083,
get = count:@80:{16},
}