South African Amateur Radio Development Trust

Tel: +27 (0)12 991 4662    Fax: +27 (0)12 991 5651

To promote the Amateur Radio Service as a national asset contributing to technology education, self-development and as a stepping-stone to a career in Electronics and Communication.



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South African Radio League



Bursaries are available for learners and students under 25 years of age to become radio amateurs. Details



John Willescroft ZS6EF 

A Sunday Afternoon Project that is Interesting to use and can be really useful.

It is a nice summers day and you are enjoying your hobby, then suddenly a thunder clap announces the start of the antenna race. Can you disconnect the HF antennas before the next bang or will you be holding the connector in your hand when the next strike occurs. We have all been there, we try and pull out the plugs before it gets to bad, but sometimes we are caught. 

This device called the Lightning Predictor it will give you a warning of an incoming lightning strike and with a little imagination and Amateur ingenuity you can use it to disconnect things before and during a storm. 

The device is very simple it measures the static charge between the clouds and the earth and as the charge builds up before a storm the led indicators shows you the charge increasing in intensity. It is very interesting to watch the wave type motion as the led signals increase and decrease, as the static charge increases and decreases,  as a storm approaches, and the sudden drop in charge when a strike occurs.

How it works 
A small telescopic or single rod antenna of approximately 1meter in length is connected to the pin of a small microprocessor together with an attenuating  resistor, zener diode, and a capacitor, all connected to the negative supply.

The rod picks up the static charge together with other electronic noises. The resistor which is adjustable makes sure the correct level can be set on the microprocessor pin. The zener diode protects the microprocessor pin from over voltage, and the capacitor slows down the fast edges of the lightning so that the zener can clamp them. 

The microprocessor measures the AC & DC voltage components on the input pin to determine when the result is between 4 set levels. When each level occurs the appropriate led is lit. No charge on the pin and the bottom led will flash every second to show you the unit is alive and the microprocessor is working. When the lower led stays alight then a small charge is present. As the charge increases the next led will light and so it carries on increasing  until the top led is alight.

When the top led is alight then a strike is immanent, by this time you are checking your rigs for damage. 

You will find an option link on the PCB LK1. When this is linked permanently the bottom led will flash each time any led is re-lit. This allows you to disconnect the led and connect a small sounder with a diode across it to the bottom led. This arrangement will give an audible warning when conditions change. The 1 second flash to show the unit is connected and working is removed so when you just hear the beep. All will be quiet until the charge builds up then you will hear the beep and  know the fun has started. 

Making it
Construction is not critical, the board can be made as the image given, else the unit can be made on Vero board. The shape of this PCB has been tailored to fit a small potting box 72 x 50 x 22 mm. The photo shows the unit fitted in the box. The rest of the construction is common sense.

 That micro controller
You will need a Pic micro controller 12F675 DIL that has the firmware programmed into it. If you have not used a micro controller before now is the time to start.  There is no escape if you want to construct a today type project you must master this process which is very easy and this is an ideal project for you to make a start.

 The Lightning Predictor is now available in kit form



How to order the kit 

Deposit R150 including  packaging and postage in the SA Amateur Radio Development Trust account.

Send proof of payment and your full name and delivery address (postal)  to

Mark the subject line LP order

Bank details:
SA Amateur Radio Development Trust
ABSA Bank Menlyn  632 005
Account no: 560 142 722