This audio level indicator was found in October 1980 Radio Electronics magazine. See the table with the dB values of the indicated signals.
Found in old English documentation this circuit can be made with a BF245 or equivalent. Coils are...
This circuit is indicated for the generation of signals in the 40 meters range. The circuit was...
Carbon microphones are no longer used in most projects. However, finding one in the scrap metal, it...