Radio with amplifier (CB20486E)

We found this circuit in a publication of 1991. The audio amplifier is an operational that admits equivalent.

Precision Optical Level Detector (CB4955E)
This circuit is from the 80's and can be implemented with modern equivalent components. R1 is 2M5,...
Light Pulse Duration Meter (CB6752E)
Indicated by Texas Instruments, this 1968 circuit measures the duration of a light pulse captured...
Liquid Level Alarm (CB5310E)
This circuit was found in a 1980s documentation. It triggers a relay when liquid from a reservoir...