This circuit for frequencies of 20 to 2 kHz was obtained in English documentation from 1966. The transistor can be any general purpose and powered by batteries.
This FET voltmeter is from an old 1974 amateur radio publication. The circuit can use MPF102 or...
This indicator was found in a 1974 publication, but it can be fitted with the BC548 transistor or...
This circuit was found in the comparator's own documentation, serving others in the same series. The...