Sound Effects Generator Circuit

The circuit consists of four parts: a binary counter, a D/A converter, a VCO, and an audio output amplifier. The speed at which the counter counts depends on the frequency of the output of the VCO, which in turn is determined by the output of the counter. That feedback loop gives this circuit its characteristic output. The initial frequency of oscillation is determined by potentiometer Rll. The VCO first oscillates at a relatively low frequency, and it gradually picks up speed as the control voltage supplied by the D/A converter increases. The D/A converter is simply the group of resistors Rl tlirough R8. When none of ICl`s outputs is active, little current will flow into the base of Ql, so the VCO`s control voltage will be low.
Sound Effects Generator Circuit - schematic

As more and more counter outputs become active, base current increases, and so does the VCO`s frequency of oscillation. The VCO itself is composed of lC2-a, lC2-b, and Ql; the timing network is D1 through D4, 01, RIO, and Rll. The diode bridge functions basically as a voltage-controlled resistor. The buffer amplifier is made up of the four remaining gates from IC2, all wired in parallel. The volume is sufficient for experimental purposes, but you might want to add an amplifier, speaker, or both.

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