This shield circuit is from a 1968s Motorola documentation. The relay is according to the supply voltage. R1 can be reduced to lower trigger voltages.
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...