■ WITH TJ BYERS
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist.
Feel free to participate with your questions,
comments, or suggestions.
Send all questions and comments to:
Hot chocolate, a warm fire,
and a list of fun, one-night
projects. What better way to
✓MIDI AB switch box.
✓Phone transmitter key.
✓Model RR trolley controller.
■ FIGURE 1
QCould you please show
me how to make a
variable-frequency sawtooth generator?
— A Devoted Reader
AYou didn’t say if you
wanted a digital or
analog circuit, so I will
give you both. A sawtooth wave is characterized by a
positive-going linear voltage ramp
concluded with a sharp drop to
zero (Figure 1a). One way to
generate a sawtooth is to slowly
charge a capacitor via a constant
current source, then quickly discharge the capacitor by shorting it
out. By repeating this process, a
sawtooth waveform is created.
But constant-current sources
can be complex — especially if
you want to make it adjustable. In
lieu of a constant current source,
a fixed resistor is often used to
limit the cap’s charging current.
However, the voltage across a
charging capacitor using a fixed
resistor isn’t linear. It starts off fast
and finishes slowly, creating the
waveform you see in Figure 1b.
But by selecting a section of the
curve that is more or less linear —