The schematic diagram below describes a simple continuity tester circuit. This circuit is used to test if  two point is electrically connected by sounding a tone, which the pitch is inversely proportional to the resistance of the tested points.  This circuit is designed for high resistance circuit, but however, this circuit can be used for low-resistance circuits. The multivibrator’s frequency is influenced by the resistance between the tested points, and this multivibrator is formed by classic flip-flop circuit  Q2 and Q1. The output stage Q4 and Q3 drive a telephone earpiece or a small loudspeaker. Here is the schematic diagram of the circuit:

Simple Continuity Tester circuit schematic