Thunderstorm Predictor

Sure, listening to VHF
FM has great advantages over MW/LW AM from the old days — now we have
bright stereo free from interference, fading and noise! However, your FM
radio will no longer predict the arrival of a thunderstorm as did the
AM radio many years ago-reliably and hours before the trouble was upon
you! The crux is that AM detection will faithfully reproduce the effects
of lightning and other massive static discharges approaching in a very
simple way: they’re audible as slight crackling noises in the
loudspeaker, almost irrespective of the tuning of the radio! Assuming no
AM radio is available anymore, a dedicated VLF receiver tuned to about 300 kHz can faithfully detect the crackle of approaching lightning.

The simple receiver shown here consists of a loosely tuned amplifier driving a kind of flasher circuit that blinks an LED in synchronicity with the lightning bolts. The frequency and intensity of the LED activity indicates the intensity and distance of the storm respectively. Looking at the circuit diagram, the LED
driver is not biased to flash until a burst of RF energy, amplified by
T1, arrives at the base of T2. The receiver works off 3 volts and has a
negligible standby current of about 350 microamperes which will hardly
dent the shelf life of a couple of 1.5-V D-size cells. T2 and T3 form a
monostable generator triggered by sudden drops in T1’s collector
voltage.

Thunderstorm Predictor Circuit

Thunderstorm Predictor Circuit Diagram

Preset P1 is adjusted until the LED remains off when you’re sure there’s no thunderstorm around for a few hundred miles. The value of the LED series resistor is subject to experimentation and LED
current. L2, C1 and the antenna are coarsely tuned for resonance at
about 300 kHz. Frequency-wise, lightning is a fairly broadband
phenomenon so any tuning to between 200 and 400 kHz will be fine for the
circuit but make sure you’re not accidentally tuned to a nearby VLF
transmitter! The input signal is obtained from a 70-cm long piece of
stiff wire, with coil L1 inserted for impedance matching and lengthening
the antenna electrically.

Warning:

This circuit and in particular the antenna must not be used to
attract lightning. Consequently, neither the circuit nor the antenna may
be used outdoors and/or powered from the mains.

Author: Karel Walraven – Copyright: Elektor Electronics

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