ATMOSPHERE_NOISE_MONITOR

Published:2009/6/22 23:21:00 Author:Jessie | From:SeekIC

Tune an unmodified transistor radio to an unused frequency near 540 kHz that's free of broad-cast-station interference; the receiver is used to pick up sferics. The received signal is fed from the receiver's earphone jack through a patch cord to the input jack(J1)of the circuit. The back-to-back audio transformers, T1 and T2, provide a suitable impedance match and signal level when the unit is used with various receivers.Diode Dl rectifies the audio input from the receiver to pulsating dc, which is filtered by C1, R1, and C2 to provide a time constant of several minutes. That dampens out fluctuations in most cases, unless lightning flashes are very infrequent.The voltage appearing at the output of the filter is a function of signal strength transferred by C1.Switch 51 is included to prgvide a convenient way to discharge the capacitors, if adjustments are re-quired during a monitoring session.Integrated circuit U1(one section of an LM324 quad pro op amp)is used as a high input-resis-tance voltmeter. Resistors R2 and R3 determine amplifier gain, and potentiometer R4 is used to ad-just full-scale meter deflection for a suitable voltage level at the input. A value of 1.5 V has been satisfactory for use with several receivers tried, but they can be changed.If the monitor is to be used only as a meter, the milliammeter can be connected directly between R4 and chassis ground, omitting R5, 53, J2, and J3. The latter components provide suitable output for use with a chart recorder having a full-scale range of either 10 mV or 1 mA. The circuit, when powered from a 9-V battery, draws about 1 mA.



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