This circuit was found in the June 1991 issue of Radio Electronics magazine. The circuit uses two Hall effect sensors of the time. It can be assembled using modern equivalents.
Using a dual 555 CMOS this very useful circuit on the bench was found in the July 1998 Electronics Now...
This circuit is from a documentation for engineers from 1977. The circuit uses a large surface diode...
Found in a Popular Electronics of the 90s this configuration can be seen elsewhere on our site....