The circuit shown was found in the 1994 National Semiconductor's Linear Application Handbook. The op amp admits equivalents and the power supply must be symmetrical.
This circuit is from an old publication from the 90s, but it can be adapted to operate with LEDs....
This squelch circuit used in transmitters was found in a magazine for radio amateurs 73 in the...
daptation of a common multimeter for the measurement of high voltages with high input resistance,...