Egor! Come quick! A storm approaches!
Here is a VLF receiver tuned to 300 kHz designed to detect the crackle of approaching
lightning. A bright lamp flashes in sychrony with the lightning bolts indicating the
proximity and intensity of the storm. Figure 1 shows the simple receiver which consists of
a tuned amplifier driving a modified flasher circuit. The flasher is biased to not flash
until a burst of RF energy, amplified by the 2N3904, is applied to the base of the 2N4403.
The receiver standby current is about 350 microamps which is nothing at all to a couple of
D cells, hardly denting the shelf life. Of course, the stormier it gets, the shorter the
battery life.
Click on image for the full.
For best effect, mount the lamp in an old-fashioned holder with an
extra-large colored glass lense. Or construct your own fixture with a plate of textured
colored glass behind a panel painted with black-crackle paint. Watch a few old science
fiction movies for other ideas.
A totally different approach is to mount the circuit in an empty glass
jar with the antenna and bulb protruding through the top. (A malted-milk jar has a nice,
red plastic lid which is easy to work and looks good.) Use a pin jack for the antenna. The
gadget looks quite home-made but fascinating.
Boat owners may wish to replace the lamp with a 3-volt beeper to provide an early warning
of approaching bad weather. Choose one of those unbreakable clear plastic jars like the
large jars of coffee creamer. A little silicone rubber will seal the antenna hole in the
lid of the jar. Use a longer antenna for increased sensitivity since there are few
electrical noise sources on the lake.
Click on image for the Full.
Tune-up is simple: adjust the potentiometer until the regular flashing
just stops. (Use a multi-turn trimmer.) When properly adjusted, the lamp will occasionally
flash when large motors or appliances switch on and off and an approaching storm will give
quite a show. Obviously, tune-up is a bit more difficult during stormy weather. Adjust the
pot with no antenna if lightning is nearby. Tune an AM radio to the bottom of the dial to
monitor the pulses that the lightning detector is receiving.
This lightning detector is not so sensitive that it will flash with every crackle heard on
the radio but will only flash when storms are nearby. Increased sensitivity may be
achieved by increasing the antenna length. The experienced experimenter may wish to add
another gain stage after the first by duplicating the 2N3904 circuitry including capacitor
coupling with the addition of a 47 ohm emitter resistor to reduce the gain somewhat. This
additional gain can cause stability problems if the layout is poor so novices are advised
to use a longer antenna or adjust the sensitivity potentiometer more delicately instead!
(When operating properly, the additional gain makes the pot adjustment much less
critical.)
