This optical tachometer with LASCR was found in a 1965 General Electric documentation. The diode at the input is an unusual type today.
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...