By Kenneth Bernholm
This little tone generator uses the reverse avalanche property of a bipolar junction transistor (BJT) with the base leg cut off. To avoid crackling when the button is pushed, I have added a resistor-capacitor (RC) bounce filter. Note that the circuit takes 18 volt. Any less and the transistor won't start oscillating. The frequency is adjusted with the R4 variable resistor or the C2 capacitor.