Make: More Electronics

Chapter 5. Experiment 5: That Whooping Sound

Instead of using a phototransistor to adjust the voltage on the control pin of the 555 timer, you can use the output from a second timer running at a much slower speed. This automates the up-and-down shifts in sound frequency.

In Figure 5-1 the previous schematic has been extended downward. (A photograph of a breadboarded version appears in Figure 5-2.) The output from a second timer is connected up to the control pin of the first timer, through a 47µF coupling capacitor. Why is the coupling capacitor there? To create a “whooping” sound. What do I mean by that? Well, you’ll recognize it when you hear it.

The second timer uses a 150K resistor to charge your choice of a 1µF or 10µF timing capacitor. ...