The $25 lightning detector
Minutes of warning before the radar app, and it still works with the internet down.
What it is
An AS3935 sensor module that listens for the electromagnetic signature of a lightning strike rather than for a weather service. It reports a strike and a rough distance estimate, out to roughly 25 miles.
The numbers
- Detection range: approximately 25 miles (40 km).
- It typically reports the first strike of an approaching cell minutes before a radar-based app does, because radar has latency and this does not.
- Module cost: $20–30.
Why it's here
If you work outdoors alone, or have stock to move, or a crew on a roof, the useful question is not *will there be storms today* but *is it close enough to stop now*. This answers that question locally, with no service and no signal.
The automation angle
Feed it to your own alerting, with a distance threshold. Strikes inside 10 miles is a reasonable 'come in' line. Log them — the strike count over a season tells you whether your surge protection is a theoretical concern or a practical one.
The catch
It is noisy. Motors, fluorescent lights, dimmers, and cheap switching power supplies all set it off, and it needs real tuning and a quiet mounting spot — often the single hardest sensor on the place to site. Treat its distance numbers as approximate. And it reports strikes; it does not report the storm that hasn't started yet.