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Lightning Detector (VLF Receiver) | Original Design
Original Design
These are the schematics I used for this project.... just in case the original website disappears like a lot of other ones, atleast there will be some referrence here.
The basic receiver is shown below. The antenna is a telescopic antenna that extends to two or three feet, the length is not critical. A high-value resistor (270k) is connected from the antenna to ground to control the Q and this value may be lowered if the circuit seems unstable but too low a value will destroy the sensitivity. The 10 mH and 1 mH ( not uH) chokes are molded types but most moderately high-Q inductors will work fine and the rest of the parts are run-of-the-mill and not particularly critical. The transistors are all general-purpose types.
Note: This circuit is intended to be used with one of the lamp options and any or all of the other options. If no lamp is desired, add a 1 k resistor from the "pulses" output to 5 VDC.
Theory of operation:
The antenna, 10 pF capacitor, and the two inductors form a resonant tank at about 300 kHz, a good frequency for receiving energy from lightning. The two series inductors act as a matching network, feeding Q1 with a lower impedance version of the signal received by the antenna. The 270k resistor lowers the Q of the resonant tank to prevent oscillation. Q1 amplifies the 300 kHz bursts and applies the larger signal to the base of a PNP transistor that forms a monostable "flasher" circuit with the last NPN transistor. When the RF signal pulls the PNP base voltage below the voltage on the 10 uF capacitor (plus about 0.6 volts) the PNP turns on, turning on the NPN. Since the NPN is connected to the base of the PNP through the 82 k resistor, the PNP turns on even harder. This regenerative action causes the circuit to turn on quickly and fully, pulling the "pulses" line to nearly zero volts. The circuit stays on until the 10 uF capacitor discharges at which point a similar reverse regenerative action causes the circuit to quickly switch off. The capacitor then quickly charges through the 1k resistor (in one of the lamp circuit options) and the diode and is ready for another pulse.
LAMP CIRCUITS
The second circuit will drive higher current lamps, up to 500mA. Flashlight bulbs make a bright flash.
METER CIRCUIT
The meter sensitivity may be altered by changing the 5.1K resistor.
SPEAKER CIRCUIT
No volume control is included but the sound level is not particularly loud.
Choose any or all of the above circuits and connect them across the indicated terminals on the basic receiver. An alarm, buzzer, or other load may be activated when the lightning activity exceeds a preset level using the following circuit:
A different op-amp or comparator may be used, as long as it is "ground sensing". Look for single-supply types for substitutes. The '339 requires a pull-up resistor on the output but op-amps will not require this part (the 1k resistor). The power supply may be the same 5 volt supply or a different voltage as long as the voltage is within the operating range of the op-amp or comparator. The VMOS transistor may be any type sufficient for driving the load. A VN10KM is a typical part for lower current loads. Add a power switch in series with the load, if desired.