acoustic ohmmeter

1 test probes are an open circuit. This permits numerous possibilities. The simplest version which uses a small transformer and a bell is shown in figure 1. The problem with this circuit is that large amounts of current can be produced which could prove too much for the circuit being tested. By replacing the bell with a small loudspeaker and suitable series resistor the current flow can be limited to below 1 mA. This is shown in the circuit diagram of figure 2. The transformer secondary voltage is full-wave rectified by diodes D1 ... D4 to provide a 100 Hz signal for the loudspeaker. When the base of T1 is connected to R1 (testing for a short circuit) the two transistors will amplify the signal to produce the required tone. There are many instances where it is necessary to test electrical con- nections, for instance, cable networks, the wiring of connectors, and the various interconnections on a printed circuit board. These tests can be carried out with an ohmmeter, but it is ofte...