This circuit uses the integrated circuit CMOS number IC 4011 pretend Square Wave Oscillator , which touch buffer change output (which compose Complementary pair of transistors) for a loudspeaker next by bass keen in the sound can control with potentiometer or 10 K ohm when we wasp the circuit suits power supply as a result , be , sound generator get , and in other words we can use to are Continuity Tester by use probe 2 reach instead of itch knock the password there. In addition Input pin , that still no use of , gate CMOS every a pin join with negative supply for feed prevent all noise.
http://www.eleccircuit.com/morse-buzzer-alarm-or-continuity-tester-by-ic-4011/
