This circuit from General Electric was found in a 1967 documentation. The SCR is of the time, as are the transistors, but equivalents can be used.
This configuration was found in the January 1992 Radio Electronics magazine. The frequency is set...
The microwave sensor of this circuit is a common ceramic capacitor. The circuit is from an 80's...
This sensor circuit for a small robot was found in a Popular Electronics from the 90s. The circuit...