This continuity tester not only show if a connection is continue or not, but also determine the range  of  its conductivity level. Here is the schematic diagram of the circuit:

Deluxe Continuity Tester circuit schematic

Finding wiring trouble is easier if we know the quality of the connection, and this continuity tester give rough estimation. Small transistors will be sufficient for this circuits since the buzzer (sound device) normally doesn’t need large current.  [Circuit’s schematic diagram source: lh-electric.4t.com]