Based on the LF256 operational amplifier, this circuit was found in the September 1989 issue of Radio Electronics magazine. The indicator is analog, and the power supply must be symmetrical.
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...