Elenco 130-in-1 Electronics Playground - Manual (Page 141)

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Multivibrator oscillators produce square waves, and
you can use square waves as test signals. You should
be familiar with multivibrator circuits from previous
experiments. The name square wave comes from the
pattern produced by the signal on an oscilloscope
(shown below).
Build this circuit and you will hear the sound produced
by a square wave signal. You can differ the pitch and
the frequency of the signal by modifying the control.
This varies the current supplied to the PNP transistor
bases.
Notes:
EXPERIMENT #119: SQUARE WAVE OSCILLATOR
Schematic
Wiring Sequence:
77-75-48-27-124
28-81-83
40-107-84
41-106-76
119-42-45-80-EARPHONE
43-105-82
78-87-108-44
46-88
47-79-EARPHONE
121-122