This circuit was found in American documentation from the 90's. The circuit applies to any operational. The power supply can be between 5 and 15 V depending on the operational.
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...