We found this circuit in a 73, magazine for radio amateurs of the 90s. The circuit is passive and most telecommunications receivers can be added.
Found in old documentation, this simple test checks both the polarity and the existence of voltage...
Found in Radio Electronics from the 90s, this circuit can vary the frequency in small values...
This circuit can be made usng the TIP32 transistor but must use carbon microphone (not very common these...