This circuit is designed to monitor the atmospheric charge. By observing the increase in the atmospheric charge, it is possible to predict a nearby lightning. The sudden electric discharge during a lightning may cause the flow of excessive current to earth which may be detrimental to the appliances connected to mains lines.
Normal atmospheric charge on a sunny day will be around 100 milli volts with negligible current and may increase to many volts as the clouds accumulate in the sky. This may go up to thousands of volts before the lightning strikes. The circuit described here monitors the atmospheric condition and display the same through LED indications.
The front end of the Atmospheric Charge circuit has a Op Amp comparator built around IC TL071. It is a JFET input op amp with an open loop voltage gain of 100dB.Its inverting input is connected to the positive rail through two presets VR1 and VR2 while the non inverting input is connected to the antenna. Resistor R2 protects the input of IC from excessive charge while R1 keeps the non inverting input stable. TL071 has very high gain, so R3 is provided to give some negative feedback.
Output from IC1 is around 2.5 to 5 volts which passes to the input pin5 of IC2 through VR3. Resistor R4 protects VR3 if its wiper is turned fully. IC2 LM3914 is a monolithic integrated circuit< that can sense analogue voltage levels to drive the LEDs and provide a linear analogue display. Current through the LEDs is self regulated by the IC eliminating the need of current limiting LED resistors. Its low bias current input pin5 receives signals down to 0.5 volts and its outputs 18 to 10 sinks current one by one as its input receives an increment of 125 milli volts. The LEDs can give a DOT mode display if pin 9 of IC2 is unconnected. If it is connected to positive rail, bar mode display can be obtained.D1 lights when the input pin 5 gets 1.7 volts andD5 light up when the input gets 4.2 volts.
Before applying power to the circuit, turn the wiper of VR1 to the extreme anti clock position and keep the wiper of VR3 in the middle position. When the wiper of VR2 turns to clock wise direction, D2 to D5 light up one by one. Adjust VR3 till D4 turns on. Wait for some time and see that D5 is glowing. This shows that the circuit is responding to changes in atmospheric charge. It is necessary to make adjustments on a sunny day with clear sky. Adjust VR2 till D1 glows. This indicates normal atmospheric charge. As the charge in the atmosphere increases due to the accumulation of clouds, D2 through D5 lights.
Antenna can be 3 meters plastic wire. The negative rail of the circuit should be earth grounded since the circuit is monitoring the atmospheric potential with reference to ground. Earth grounding can be done by piling an iron rod into the earth. Connect the circuits negative to this iron rod. Keep the unit inside the room near the window. Do not try adjustments if there is lighting.