Everyone knows that lightning can kill and certainly destroys billions of dollars of property every year. Data from NOAA, The National Oceanic and Atmospheric Association, indicates that lightning is the 2nd leading cause of storm deaths in the US with about 100 each year. In addition, lightning causes about 1000 injuries and $5 billion of damage each year -- and that’s just the US.  For outdoor enthusiasts, the large, expensive, ac-powered detection systems used at airports and sports arenas are no help at all. And the so-called portable devices available today are bulky, expensive, power hungry, and are subject to frequent false triggering from common noise sources, such as fluorescent lights, microwave ovens, a car starting, etc.

The human senses are not well equipped to perceive the onset of a lightning storm. The furthest distance that humans can hear thunder is about 10 km, and that’s with a quiet environment without any physical obstacles baffling the sound. However, if there are physical obstacles and / or a high level of noise (e.g. traffic, people, etc.) the maximum distance is decreased to sometimes just a few km. That is why the 30/30 Rule was established -- go inside if you hear thunder within 30 seconds of a lightning flash; wait at least 30 minutes after you hear thunder before going back outside.

But it gets worse. Lightning typically does not strike to ground in a vertical fashion but rather diagonally, covering distances which might go up to 10 km. This clearly indicates that it’s possible to barely be able to hear a lightning strike yet it can strike right where you stand.

Electro-Magnetic Propagation (EMP) in lightning

As early as the 18th century, Alexander Stepanovic Popov noticed that it was possible to detect lightning using a simple AM radio receiver. This was the first electrical system capable of predicting a storm. He used a simple system with an amplifier, down-mixer and a low-pass filter and was able to hear the signal produced by lightning.

Similar technology is still in use today in portable consumer lightning detectors. Although the American Meteorological Society does not recognize the reliability or value of these portable detectors, these device scan, under the right conditions, detect lightning within a small area. However, these rudimentary devices cannot estimate the distance from the head of the storm, nor can they reliably differentiate lightning from interference.  Common interference sources include microwave ovens, fluorescent ballasts, motors, car engines and other electrical disturbers. In addition, these systems are based on discrete components and use a lot of power. As a result, battery life is limited to a couple of weeks.

This discussion highlights the need for a personal device that accurately estimates the distance to a storm and reliably identifies lightning strikes.

Detecting lightning with a narrowband receiver

Lightning strikes in two different ways: cloud-to-ground and cloud-to-cloud. In terms of electromagnetic analysis, the huge currents generated in storms produce a unique signature. However, monitoring such a signal can be difficult with a portable consumer device. Popov’s experiments fortunately proved that a narrowband system could indeed sense signals from lightning, and with pretty good accuracy.

A new sensor for lightning detection

Such a narrowband receiver has now been implemented in ams’ AS3935 Franklin lightning sensor. This technology is effective for both cloud-to-ground and intra-cloud lightning.  It uses propriety algorithms that analyze incoming signals and compares their shape to atypical lightning strike’s waveform. Exhaustive effort has gone into tuning the algorithms so that it provides an optimal balance between lightning detection and interference rejection.   In addition, the Franklin sensor derives accurate distance estimations by analyzing the signal energy detected by the IC’s RF front end.

Figure 1 shows a block diagram of the Franklin. Like the Popov system, it monitors the LF bands to detect the strong 1/f signature characteristic of lightning. The system includes an Analog Front-End (AFE) to filter and amplify the input signal and then transfers it to the baseband. The lightning algorithm block consists of three stages: signal validation, energy calculation and statistical distance estimation.

Fig. 1 block diagram of the ams AS3935 Franklin lightning sensor IC

The first block checks the pattern of the incoming signal and identifies it as a real lightning signal or a man-made signal. Atypical lightning signal goes high very fast and declines smoothly after the peak. The IC’s algorithm allows optimization by the end user of the trade-off between lightning detection effectiveness and disturber rejection.

If the received signal is identified to be lightning, the second block performs an energy calculation. This calculation is then analyzed in the last block. This section makes an assessment of the distance to the head of the storm based on data collected during the storm.

This unique highly integrated solution is the world’s first Lightning Sensor and its 60uAlow-power listen mode will allow for extended battery life using single coin cells. An end product requires just a few additional components: a simple microcontroller, an antenna and some passive components. 

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