Solid state relay (CB8518E)

This circuit was found in a 90s Popular Electronics. Both the coupler and the MOSFET can be of any type.

100 kHz calibrator (CB12372E)
I found this circuit in Popular Electronics in July 1991. It generates an accurate 100 kHz signal,...
Metronome using operational (CB22060E)
We found this circuit in an English publication. The transistor can be BC549, and the power supply...
Voltage to frequency converter (CB20607E)
This circuit makes use of components from the 80s. However, the configuration used can be used as a...