A continuity tester can be achieved using this circuit diagram. The continuity tester produces a sound when the connection that is checked is not interrupted.
Circuit is very simple, requiring few electronic components.
Bridge rectifier produces unfiltered voltage that can be notified into a loudspeaker as a sound of 100 Hz. This sound is produced when the transistor T1 receives a current through resistor R1 and tested connection, causing the transistor T2 entering in conductivity.