The circuit is from 1968, but we can use a modern one like the 741. The symmetrical power supply voltage (two batteries or source) can be from 9 to 12 V.
This two-color LED indicator was found in a Popular Electronics from the 90's. The transistor...
This modulated circuit was found in the November 1997 Electronics Now magazine. The power...
This circuit is primarily intended for the calibration of common radio IF steps using the...