ICl, a 555 timer, is set up as an astable multivibrator to produce the signal that triggers IC2, a 7490 decade counter. That IC, in turn, produces a BCD output that is fed to IC3, a 7445 BCD-to-decimal decoder/driver. IC3`s output is inverted by two hex inverters, IC4 and IC5. The outputs of IC4 and IC5 are inputted to control pins on IC6 and IC7, CD4016 CMOS quad bilateral switches. As those switches open and close, different resistances (as set by potentiometers R3 through RlO) are inserted into the sound-generating circuit made from ICS.
Electronic-music - schematic

The frequency atthe outputs of IC6 and !C7 are adjusted to various rates, using potentiometers R3 through RIO, to produce the desired tones. Capacitors can be placed in series with the potentiometers to produce a sloping sound instead of a straight tone. The negative-going output signals of IC6 and IC7 are fed through a common bus to pinS of ICS.

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