Weathered Radio

A while ago, I wrote about listening-in to our wireless water meter with a USB SDR. A while after that, we received a weather station that includes a wireless sensor to measure the weather outside:

With the excellent rtl_433 utility, it was quite easy to get a reading off of the wireless sensor. The following script will launch `rtl_433` for 30 seconds and output the readings in JSON format. We then use `jq` to select only the readings specific to our sensor by filtering by sensor id (there are a couple other wireless sensors around it seems), keep only the last reading, do some calibration with `jq` again and write the final JSON to a file. I then point nginx to that file, serve it with a 'application/json' Content-Type and boom: local weather accessible remotely. Why? No idea, but it's kind of neat.

#!/bin/bash

# output dir
dir="/tmp"


# Run rtl_433 for 30 seconds (the sensor returns data every 16s, but whatever)
/usr/bin/rtl_433 -R 40 -F json -T 30 | \
# Filter out everything we received except data from our sensor (id=2168)
jq --unbuffered -c 'select(.id == 2168)' | \
# Only keep a single reading
tail -n 1 | \
# We subtract "3" from the temperature reading since the sensor seems to
# consistently be around 3 degrees too high
jq --unbuffered '.temperature_C = .temperature_C - 3' > "${dir}"/weather.new.json

# Overwrite the old file with the new data
mv "${dir}"/weather.new.json "${dir}"/weather.json

I use the following systemd timer/service to have the script above run every 15 minutes:

[Unit]
Description=Update weather info every 15mins

[Timer]
OnBootSec=15min
OnUnitActiveSec=15min

[Install]
WantedBy=timers.target

[Unit]
Description=Update weather info

[Service]
Type=oneshot
ExecStart=/home/chimo/scripts/weather.sh