Death by lightning is rare in the United States, but the worldwide danger is real. Lightning occurs 50 times per second around the globe and causes 20,000 deaths per year and more than 200,000 injuries.
It strikes even the experienced outdoorsman: Ian McKeever, who held the world record for the fastest completion of the Seven Summits Challenge (the highest mountains on each of the seven continents), was killed by a lightning strike on Mount Kilimanjaro in 2013.
Dedicated lightning detector devices exist, but they tend to be bulky and are often forgone in favor of packing light for hiking or camping. The AS3935 from ams AG, which comes in an MLPQ 4x4 package, is a lightning sensor ideal for outdoor and portable devices. It has an early warning indicator effective to within 40km of storm activity, and even provides distance estimation to the head of the storm. It detects both cloud-to-ground and intra-cloud flashes and integrates an intelligent algorithm for false disturber rejection, all in a comparatively small form factor.
From a hardware point of view, the AS3935 only needs a loop antenna in parallel to an RC circuit. The antenna has to be designed to have its resonance frequency at 500 kHz and a quality factor of around 15. The signal generated by the antenna is then amplified and a proprietary lightning algorithm analyzes the received signal.
The lightning algorithm has 3 distinct blocks:
- Signal validation: verification that the incoming signal can be classified as lightning. If it is not, the next two tasks are not performed. The shape of the incoming signal is analyzed, allowing the sensor to differentiate between signals that show the pattern characteristic of lightning strikes and man-made disturbers like random impulses. By setting the spike rejection level, users can program in an optimization of efficiency with regards to false signals.
- Energy calculation: calculation of the energy of the single strike.
- Statistical distance estimation: a distance is estimated according to the number of stored strikes. This assessment is done based on statistical calculation. The output of the energy calculation block is stored along with timing information in an internal memory. All the events stored in the memory are correlated with a look-up table to provide a kilometer distance estimate to the head of the storm.
The current is also optimized for wearable applications. A power down is available that consumes only 1µA. In listening mode, it is possible to configure the AS3935 to consume down to 60 µA and the verification mode only requires 350µA.
With the current boom in high tech sport equipment and smart clothes with built-in sensors, a compact, wearable lightning sensor is well within mainstream reach. One thing is for sure, it would undoubtedly save lives.