This circuit uses three inverters to generate a rectangular signal whose frequency depends on R1 and C1. The circuit is from a March 1996 Electronics Now.
Other CMOS gates can be used in this sensor, which we found in the July 1995 Electronic Now...
This circuit is from the American magazine that no longer exists Science & Electronics,...
Using the 4046 PLL we found this circuit in the December 1989 Radio Electronics. The circuit...