Sound effects generator

  
This circuit can generate a European police-car siren, bird noises, spaceship sounds, etc. In addition, it can be put to use as a doorbell, an alarm, etc. The circuit consists of four parts: a binary counter, a D/A converter, a VCO, and an audio output amplifier. The initial frequency of oscillation is determined by potentiometer Rll. The VCO first oscillates at a relatively low frequency, and gradually picks up speed as the control voltage supplied by the D/A converter increases.
Sound effects generator  - schematic

The D/A converter is the group of resistors R1-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. As more and more counter outputs become active, base cutrent increases,and thereby so does the VCO's frequency of oscillation. The VCO itself is composed of IC2-a, IC2-b, Ql, and the timing network comprisingD1-D4, Cl, RIO, and Rll. The buffer amplifier is made up of the four remaining gates from IC2, all wired in parallel.




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