The circuit diagram of Fig.1 is a very sensitive static electricity detector that can provide an early warning of approaching storms from inter-cloud discharge well before an earth-to-sky return strike takes place. An aerial (antenna) formed of a short length of wire detects storms within a two mile radius. The circuit emits an audible warning tone from a piezo buzzer, or flashes an l.e.d. for each discharge detected, giving you advance warning of impending storms so that precautions may be observed, such as unplugging modems, switching off computers and so forth.

The primary feature is the circuit’s ability to be set close to self-oscillation, with its relaxation optimised via the bias resistor values shown in the circuit diagram. The oscillator is d.c. coupled and feedback is routed through the collector (c) of transistor TR1 to the base (b) of TR2, while the overall loop gain is set with the multiturn (12, 18 or 22) preset VR1.

Capacitor C3 sets the fixed phase at the emitter (e) of TR2, and at the wiper of VR1 capacitor C2 increments the phase shift for oscillation to occur. (Any similar small signal high-gain transistor can probably be used, e.g. a BC548C – ARW)

The collector of TR2 outputs a 42kHz sinewave once triggered, and is coupled by capacitor C4 to the base of TR3. Diode D1 rectifies the positive-going phase of the oscillator, to bias TR3 on and output a d.c. voltage at TR3 emitter. This drives WD1, a self-contained piezo-electric sounder. A pulsed l.e.d. D2 can be added as an option for a visual indication if required.

Setting Up

To set the circuit up, adjust preset VR1 for oscillation by monitoring test point TP1, which should be at roughly 7V peak-to-peak. Test point TP2 should be at +6V d.c. Now readjust VR1 back slightly to stop oscillation; use a screwdriver to touch the aerial-side of C1 several times; the alarm should sound for 1 or 2 seconds then stop. If it continues, make a very small adjustment back, and recheck. The other method is to electrostatically charge a plastic ruler, and then draw your finger close to discharge, about two metres away from the aerial.

Powered from a 9V battery, the Lightning Detector circuit consumes about 600
A in standby. Powered continuously it could provide a good year of uninterrupted monitoring. When sounding the alarm, the current will rise to 4mA depending on the low current sounder WD1. A minimum 3V device is required for a good output level, and it will produce a “pinging” alarm to warn in real time of any electrostatic pulse activity.

Article reproduced by permission of Wimborne Publishing. www.epemag.com