by Steve Ciarcia & Jeff Bachiochi

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Start • It's All in the Grounding • Transient Voldatge Supression • Modem and Power-Line Protection • Unplug the Computer! • Automatic Thunderstorm Switch • Black Box It • Waiting for Summer • Sources

AUTOMATIC THUNDERSTORM SWITCH

We can watch for rain, listen for thunder, and count the seconds after seeing the flash. These are the obvious indications of a threatening situation. There are many less obvious indicators as well.

The energy propagated from the current flow of a lightning strike contains wideband energy. Everything from 100 Hz to 100 MHz is produced.

Emissions below 100 kHz travel along the wave guide formed by the earth’s surface and the lower ionosphere. With respect to the earth (ground), the air around the strike becomes charged, and there is a direct relationship between the amount of charge and the distance from the strike.

We located a minimum-cost lightning sensor from McCallie Manufacturing. Of the two models available, we chose the LSU2001, which is priced around $50.

Simple circuits are also provided for adding a meter or LEDs to monitor live data, or you can connect the sensor through an optocoupler to a PC. Optional software lets you count and graph storm data (providing you wish to keep the PC on day and night).

The manufacturer suggests mounting the LSU2001 on a well-grounded metal pole. The higher above ground it’s mounted, the farther away you’ll be able to detect lightning strikes.

Sensitivity is related to the differential charge between the air and ground—about 0.15 V/m. Put it twice as high, and it will be twice as sensitive.

They suggest that setting it 5' high covers 15 miles, 10' covers 50 miles, and 25' covers 150 miles. The latter is enough to cover all of Connecticut, Rhode Island, and Massachusetts from our location.

Jeff wasn’t enthusiastic about erecting a pole in his yard, and I wasn’t volunteering to make like the Statue of Liberty.

However, since both of our roof peaks already had well-grounded lightning rods installed, attaching the sensor there made the perfect compromise location (see Photo 3).

The sensor has two wires leading out of its plastic enclosure. A coaxial connector would have made this a much cleaner job.

The coax was soldered to the wires and covered with tubing to make it watertight. The coax needs to be earth grounded so the sensor’s internal circuitry can operate properly.

Interestingly, the sensor documentation comes with more warnings than any other piece of apparatus we’ve seen lately. Perhaps rightly so. If there’s one thing you don’t want, it’s to provide a direct path for lightning into your house.

It’s strongly recommended, for this reason, that the sensor’s signals be isolated optically from your equipment and powered by its own battery. This setup also prevents line noise from interfering with the sensor. Figure 2 shows how it’s done.

Battery longevity is essentially its shelf life. Power is only consumed when the air-to-ground potential rises above ~1.4 V. When this happens during a lightning strike, the circuit produces a pulse that flashes an infrared LED (LED1).

The IR LED points at a phototransistor directly or via a fiber-optic connection. The LED and phototransistor combination functions as an optoisolator. The greater the distance between them, the greater the isolation protection.