You will be aware of how a domestic Medium Wave (BC Band) radio crackles when storms are near. This is due to RF radiation from the lightning, which occurs during the actual lightning stroke when rapidly varying currents of several kilo-amps (kA) sweep along the lightning channel. The radiation is predominantly at very low frequencies (VLF) and is commonly termed 'sferics', an expression which comes from the earliest days of radio and is a shortening of 'atmospherics'.
The best way to detect them is to tune a radio receiver to an empty frequency somewhere in the LF region, and for small portable devices a frequency around 40 kHz is fine. I first built a device of this type in the late seventies, and it became called 'Lightec' for lightning detector. This had purely a visual display, an LED that got triggered on for a second whenever a sferic was received. The success of this device led to the development in 1984 of Lightec AV mk2 shown in the photos below. It operates on 40 kHz, and has both an LED and a speaker so you can listen to the sferics- a strangely relaxing exercise. You can select either mode, hence AV for audio-visual. The unit is powered by rechargeable cells and is built in the case of a pager, using the pager's audio transducer as a speaker.
This little box is 24 years old now and still going strong; and it had an overhaul in 1994 and new cells fitted- but you can't get these 90 mAH DEAC cells any more so I'm considering converting it to use lithium coin cells instead. But a more up to date project is underway as well, to provide a microprocessor controlled device that will provide lots more information.
In the late 1980's I built a modified version of the Lightec where instead of flashing an LED, the monostable circuit was modified to pulse an electromechanical counter. Parts of this unit still exist, however trials came to an abrubt end when, during battery charging using an accidentally way too high current, one of the battery cells exploded blowing a hole in the side of the case and largely destroying the electronics. I'm just glad I wasn't around at the time. However, during it's life, some really interesting results were obtained. The unit was sited in an outbuilding to cut down on false counts from thermostats etc and I used to read the counter each day, subtract the previous day's count and plot the result on a graph.
I started work on replacement a few years ago, and the idea was to log not just the total count but also to provide a history so that the user could see whether activity was increasing or decreasing. The prototype uses a three-stage radio receiver tuned to 40 kHz (no surprise there then) and this feeds into a PIC16F84 microcontroller, which does the counting.
I must emphasis that this is a prototype device, and the display isn't so user friendly but in the picture above, the top line is showing the total flashes recorded, and the bottom line is showing the history. This is a row of digits, each representing a minute ago in time. Up to 9 counts in a minute can be counted, and after one minute this is passed to the digit to the right, then to the one to the right of that a minute later etc so a history over the past ten minutes is displayed. In the photo, it's showing a total of 6 counts, with 1 three minutes ago, 1 five minutes ago, 2 seven minutes ago, and 1 ten minutes ago. A further count has dissapeared off to the right. The complete system is shown below. The consumption is about 5mA at 9v so the MN1604 battery shown (manufacturer quotes 550 mA/H) should run the unit for 110 hours which is plenty enough for casual use.
I hope you enjoyed reading this, if you've any comments or questions I'd be glad to hear- just email me at alancordwell[at]blueyonder[dot]co[dot]uk- with the things in square brackets replaced by dots and ats etc...
Last Modified 10/8/2008