BY DAVE SIEGEL
Surface-mount ICs and other electronic
components are continually shrinking in
size. This trend is reaching the point
where basic meter measurements on
ICs are a major challenge. Once the IC
pin of interest is located (usually requiring
magnification), more often than not,
the test probe is too large to make the
measurement without the risk of short-circuiting adjacent pins. Because
conventional meter probes have changed little in size from the vacuum tube
era, they are ill-suited for working with today’s microscopic-size devices.
Probing a . 5 mm pitch surface-mount IC pin with a 2 mm probe invites disaster.
My work in electronics to a high degree involves fine-pitch surface-mount devices and, because of
this, I frequently encounter the test probe/device size
mismatch. A Google search showed that micro-sized test
probes are available, but they are quite specialized and
usually very expensive. I decided to design and build my
The initial design consisted of a long embroidery
needle with an attached wire lead. The opposite end of
the lead was terminated with a standard banana plug.
Heat-shrink tubing covered 90% of the needle, with only
the tip exposed. The drawback of this simple design was
that the finished probe was very difficult to hold and
caused hand fatigue. Clearly, the needle required a
suitable housing to be of practical use on a test bench.
A brainstorming session with paper and pencil
produced the answer: Use a mechanical pencil for the
housing of the micro-probe. The probe tip would, like
the pencil lead, be retractable — a great safety feature.
The design was finalized and a prototype soon followed.
■ FIGURE 1. Micro-probe schematic diagram.
After several months of use, the micro-probe has proven
itself an indispensable part of my test bench and feedback
from colleagues using the probe has been very positive.
I hope Nuts & Volts readers will enjoy building the
As the schematic in Figure 1 shows, the circuit is very
simple since there are no electronic components involved.
The micro-probe is easy to build and can be completed
in a few hours. I’ll provide a very detailed assembly
procedure for the benefit of less experienced hobbyists.
The dimensions given in the assembly procedure are
specifically for the Pentel “Side FX” mechanical pencil,
because the Side FX is inexpensive (about $2 each) and
readily available. Other pencils will work, but wire length
and needle dimensions may need adjusting. The selected
pencil should have a side lead-feed button rather than
a top feed because the pencil top is the exit point for
the lead wire and must remain unobstructed. Figure 2
identifies some of the Side FX components and it will be
a useful reference during construction.
To begin, remove the lead from the pencil, then
separate the top and bottom barrel sections by pulling