Power Grid Monitor (CB21154E)

We found this circuit in a Popular Electronics from the 90s. Its purpose is to monitor a power line. The circuit uses optical couplers of the time.

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Automatic gain control for audio (CB20640E)
This circuit was found in an American publication called Ham Radio. The circuit uses still current...
Phase Splitter (CB4239E)
From an audio signal, this circuit gets two signals of the same amplitude and opposite phases....
Low frequency linear modulator (CB7305E)
Found in an Electronic Design from 1968 this circuit generates a linear sawtooth signal modulated...