Telephone Busy Indicator (CB24430E)

This circuit for analog telephones from the 90s was found in documentation from the time. It lights up a remote LED when the extension is in use.

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Sine to Square Converter (CB6586E)
Found in American documentation this circuit uses transistors of the time. The circuit can be made...
Electronic flasher (CB23766E)
This flasher with two power transistors was found in a Radio Electronics magazine from September 1961....
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Found in American documentation from 1969, this circuit switches signals with a FET of the time....