Receiver for FM Infrared Audio Link (CB20978E)

This circuit was found in a Popular Electronics from the 90s. The components admit equivalents and the phototransistor can be of any type.

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Intercom with telephone (CB7829E)
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...
Precision Light Alarm with 741 and CMOS (CB25486E)
This circuit was found in the July 1992 issue of Radio Electronics magazine. The circuit has a...
Inverted Polarity Indicator (CB16580E)
This simple circuit indicates if the polarity of a load is reversed when it is connected to a...