The constructionis the multivibrator with one condenser and two complementary transistors.The PNP (BC557 or 2N3906) can only switch through when the NPN transistor ( BC547 or 2N3904)opens. The latter receives its basic current by means of a time link, consisting of the condenser 100 nF and the resistor 27 kΩ. This combination is useful when working with a loudspeaker of 8Ω. Sound switchers of other impedances as well as other pitches require different construction element factors.

When closing the switch, a working voltage of Ub = 4,5 V applies. The condenser can charge. When reaching the threshold for the NPN transistor, the latter starts to switch through and to set free the basic current for the PNP transistor. This one switches through as well and a sound of a certain frequence is being heard.
The frequence is the number of oscillations (switch on - switch off time or "oscillation periods") per second.
A high tune ( a high frequence) can be obtained when diminishing the resistor and condenser values. An increase of those values produces a lower-pitched tune.

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