44 September 2016
power-down between switching in different values.
In the switch shown, I used only 1/4W resistors for
better illustration. In the actual completed unit, I used
1/2W on the upper ranges and 1W on the lower ranges —
all 1%. I also went with a 12 position switch because I
could not pass up the
price. This is a make-before-break switch
and has no stop, which
is kind of neat since
you can run through
the positions in either
direction no matter
what location it is on.
The cost per populated
averaged $1.30. Total
cost for six decades,
box, and knobs was
about $18. The
completed unit is
shown in Figure 10.
The upside of
rotary switching is
specs can be extended
over the digital type
making for a more
rugged and versatile
unit, adding an extra measure of safety for those “OOPS”
moments. Also, they are faster and easier to switch. In this
style, you will definitely want to go with 1/2W/1%
resistors, maybe upping that to 1W on the 1s, 10s, and
possibly the 100s ranges.
The downside is a bulkier and somewhat more
expensive unit than the digital style.
I have seen rotary switches priced at less than $3
from the major suppliers to as low as less than $1 from
eBay and Tayda Electronics (and they were make-before-break style). Having used all three switching styles
presented here, the rotary version is my first choice by far
for the reasons already stated and are just a pleasure to
I’m not going to dwell much on the slide switch type.
Construction is simple and straightforward, and a
schematic is included in Figure 5. The upside on this style
is it’s very economical to build for high voltage and high
capacity operation. The downside is a lot of panel
fabrication is involved and it’s a real pain to operate.
The Capacitance Decade Box
Next up is the capacitor decade box. These will be
more expensive to build. Again, starting with the digital
type of switching, most advantages/disadvantages apply
here as its resistive counterpart. For the digital style of
switching, the BCD type is the preferred switching mode
here — especially since all caps are connected in parallel
to a common bus. This will use four base values of
capacitance in a 1C, 2C, 4C, 8C arrangement.
FIGURE 6. Capacitance
box digital switch.
FIGURE 5. Slide