This circuit supplies a continuous voltage at its output, which is the peak value of the alternating input voltage. The circuit is from a 90's EDN, making use of Maxim integrated circuits of the time.
This digital instrument input circuit was found in the September 1989 issue of Radio Electronics...
In this circuit of a 1970s publication, data from the coil to oscillate at the desired frequency...
This circuit is from an old Elektor using components that are not currently common. The circuit is...