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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Spy Transmitter (CB19424E)
This small crystal microphone transmitter was indicated in an 80s spy book. The range depends on...
Recording head amplifier (CB24226E)
The figure shows how to use an operational amplifier with a low impedance head. The gain is...
Voltage Comparator Shield (CB4588E)
Voltage comparators, such as those in the LM139/239/339 series, consist of devices that can be...