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.
This circuit is from the SGS manual of linear integrated circuits of 1982. The L78xx is equivalent to...
This low displacement circuit was found in American documentation from the 90s. Operational...
The purpose of this circuit is to maintain the liquid level of a reservoir between two values...