Elenco 130-in-1 Electronics Playground - Manual (Page 143)

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This circuit emits a sound if the material you are
checking transmits electricity. This is convenient when
you are looking at wires, terminals, or other things and
cannot look at a signal lamp or LED. Your ears will
detect the results of the test while your eyes are busy.
If the component or circuit you are testing conducts
electricity, it will complete the circuit for a pulse-type
oscillator. You can use this to test most of the
components in this kit. For diodes and transistors,
remember that electricity only flows through them in
one direction (unless they are damaged).
In the schematic, you will see that the output from the
transistor goes through the transformer to the 0.02
μF
capacitor and then to the base of the transistor. The
TEST terminal is connected to the emitter of the
transistor. The transistor starts to oscillate when
something that allows electricity to flow is connected
to the terminal.
You can safely check almost any component with this
continuity checker because it uses a very low current
of about 15mA or less. You might want to try
measuring the continuity of pencil lines on paper,
water, metallic surfaces, and many other things.
Notes:
EXPERIMENT #121: AUDIO CONTINUITY TESTER
Schematic
Wiring Sequence:
1-29
2-30
3-103-109
4-87-120
5-110-41
88-104-40
42-116-PROBES
115-131-PROBES
119-132