This circuit is from a 1975 RCA tube manual. It appears elsewhere on the site. R5 depends on the voltage of the amplifier with which it is used and can be changed.
The circuit presented is quite simple, employing tone modulation of the signal. To receive the...
This circuit was obtained in a 1967 Raytheon manual. The company's component is an optical coupler with...
This circuit was obtained in an Electronic Engineering of the 90s. The operational amplifiers and...