Found in an Electronics Now from the 90s this mixer has several inputs. The integrated circuit can still be found easy and the power supply need not 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,...