LED VU (CB22157E)

This circuit was found in a 1989 publication. The operational amplifier supports equivalents. The LED range can be expanded.

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Signal tracer (CB23671E)
This 4-transistor signal tracer or follower was found in Radio Electronics magazine from May 1960....
Optical control notch filter (CB6717E)
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
Micro Spy (CB21872E)
Assembled with SMD components this circuit can be very small. The range is a few tens of meters. We...