■ PHOTO 5. The finished unit. Power and
output jacks are on the back and cannot
■ PHOTO 4. The analog board is just a dual
op-amp and a handful of components.
sophisticated filtering circuit. A single
R-C filter is quite limited. You could
cascade two or three filters in series
(which would require a multi-pole
switch). Adding a jack that allows an
external clock to drive the system is
also a useful feature, as is buffering
the “Zero” output of the first counter
(U2 pin 3) and using it as a sync pulse
for your oscilloscope.
It is possible to build two ARBs and
interconnect them so that they are
phase locked. In this way, you can have
a two-channel ARB (useful for stereo).
The simplest way to do this is to piggyback a second step circuit to each of the
existing step circuits. These added step
circuits would then be summed and go
to a separate/additional analog output.
Of course, there is no need to use
25 steps. You can increase or decrease
the number to suit your application.
An arbitrary waveform generator
is a useful instrument to have. Most
commercial units are expensive and
have thousands of steps. The simple
analog ARB described here has much
less capability than a commercial unit.
However, for about $20 in parts, it can
PARTS LIST provide reasonable performance in theaudiorange. NV
Resistors (1/4 watt)
❑ R1, R2 5.1K
❑ R3, R4 100K potentiometer
❑ R5 1K
❑ R6, R8 10K potentiometer
❑ R7, R9, R10, R11 10K
■ PHOTO 6. A typical unfiltered output.
The large glitch in the middle is
deliberate the small glitches between
the steps are not.
Capacitors ( 25 volts or higher, ceramic unless otherwise specified)
❑ C1, C7 10 pF
❑ C2, C8 100 pF
❑ C3, C9 0.001 µF
❑ C4, C10 0.01 µF
❑ C5, C11, C14 0.1 µF
❑ C6, C12 1.0 µF monolithic
❑ C13 10 µF tantalum
❑ U2, U3, U4
CD4069 hex inverter
CD4017 decade Johnson counter
CD4081 quad AND gate
LMC6082 dual op-amp
Step Circuit (One set of parts is needed for each step. As drawn, 25 of each are
■ PHOTO 7. This is the filtered result of
Photo 5. Better performance can be
achieved with something more than a
simple R-C filter.
1N4148 small signal diode (or equivalent)
10K potentiometer (see text)
10K 1/4 watt
2N2222 transistor (or equivalent)
❑ Swt1, Swt2
One-pole, six-position rotary switch