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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Overvoltage protection (CB8360E)
Found in a 1960 General Electric documentation, this crowbar circuit blows the fuse in case of...
Crystal Tester (CB24174E)
This circuit was originally designed for testing crystals used in color televisions, but it works...
Test circuit with bridge (CB22530E)
This bridge-biased transistor test circuit measures gain in the range 0 to 300. Circuit is from...