As nice as Muratore’s
design works, however, the
employed in his design are
notorious for their
sensitivity. That means you
have to hook up your
ohmmeter and adjust the
knobs back and forth
several times in order to
get the exact resistance
you need. What I really
wanted was a decade box
where I could select the
required resistance with a
couple of switches —
without having to hook up
an ohmmeter to see the
actual resistance value. To
solve that problem, I
designed the digital decade
box presented here.
This is a great project
for the beginner. You’ll get
to sharpen your soldering
skills for your next project.
What follows is the
construction and operating
instructions you’ll need to
build and operate the
The circuit for the
decade box is shown in
Figure 1. You’ll notice that
in order to select a
resistance value (440.520K,
for example) we select the
appropriate DIP switches in each column according to the
value we need.
This completes the circuit, and the total resistance of
all the switches will be available at the output (A, B). It’s
important that you set all DIP switches that
are not selected for a resistance value to
zero ohms (switch 10). In other words, if
you select 10K for a resistance value, all the
other DIP switches must be set to zero ohms
(switch #10 - ON). Remember, you have to
complete the electrical circuit in order to get
a 10K resistance at the output (A, B).
On the other hand, don’t forget to move
the zero switch to the off position for any
DIP switch selected for a resistance value.
The layout for the circuit is shown in
Figure 2. Place the DIP switches on the top
side of the perfboard (no solder pads) and all
the resistors on the bottom side of the board
(solder pads). The board is small, so count
the holes carefully. You should see two holes
between each DIP switch. Insert DIP
switches #1and #2. Place them on the top
side of the board (no solder pads).
Before you start soldering anything,
February 2016 25
■ FIGURE 3.
■ FIGURE 1.
■ FIGURE 2.