Building Projects? Designing PC Boards? There is One Ruler You Should Have

I’ve been building electronics projects and designing PCBs since high school. Those experiences led me to a career in engineering where I built more projects and designed more complex PCBs. For my last engineering gig, I worked at Cadnetix, an early PCB CAD startup located in Boulder, CO, where my focus was entirely on designing workstations and systems dedicated to the design of PCBs.

Throughout all of this work, a few minor non-electronic design challenges cropped up regularly. It seems these challenges are likely to be widespread, and chances are you, too, have experienced some of them.

For example, I often wondered about the size of some common objects such as wire gauge and bolt size. With the advent of big box hardware stores, you can often find a tool fastened to the shelves in the hardware section to help with bolt size, but it’s somewhat inconvenient to hop in the car every time you need to size a bolt.

Similarly, I always had to guess the gauge of a wire when using my wire strippers. “Is this a 10, 12, or 14 gauge wire?” I often pondered. It’s not a deep question, but it was a frequent one. A good percentage of the time, I guessed wrong and accidentally nipped off a few strands of the wire during stripping. How irritating.

Recently, I snagged a really useful and inexpensive tool to help me find answers to these deep questions, and perhaps a few more closely related to PCB design. That tool is the Digi-Key 12-inch PCB ruler (Figure 1). I’m not sure whether it’s called PCB-Ruler-ND because it helps you design PCBs, or because it’s made from PCB material. Perhaps the name is intentionally dual use.

Figure 1: The Digi-Key 12-inch PCB ruler goes way beyond inches and millimeters into wire gauge measurements, pad sizes, screw diameter, and much more. (Image source: Steve Leibson)

In any case, the Digi-Key ruler does the normal things a ruler should do: it measures in inches and metric. It has a handy straight edge for drawing lines. But any ruler can do that. The Digi-Key ruler also answers the above PCB and design questions and quite a few more.

First, the Digi-Key ruler is full of really useful holes. There are holes for sizing English (#2 through ¼ inch) and metric (M2 through M6) bolts. If you can’t tell the difference between an English and a metric bolt, that’s OK because the Digi-Key ruler can. These holes are also useful when you need to design bolt holes in your PCB.

A different set of holes answers the question about wire gauge. The wire gauge holes span the range from 10 to 30 AWG and conveniently give the metric diameter of these wires as well.

The ruler answers both common and uncommon questions about PCB design. For example, because the ruler is manufactured exactly like a PCB, it has footprints for several popular SMT packages etched into its surface, including footprints for eight and four pin SOICs and TSSOPs, SOT-23 packages, DPaks, and a few more. The ruler also has footprints for many two lead SMT components such as resistors, capacitors, and diodes. PCB traces labeled with widths in both English and metric sizes are etched along the ruler’s edge allowing you to compare and estimate the trace widths used on an existing PCB.

Two convenient charts silkscreened onto the ruler give you the current carrying capability of both English (on one side) and metric trace widths (on the other), and list the temperature rise you can expect for the amperage you push through them. Sure, you can look this information up in a book, but having it handy on a tool you keep on your desk all the time is much more convenient.

In fact, there are so many features on this ruler that Digi-Key has published a 13 page data sheet, instruction manual, and revision list for it. That’s a lot of information packed into twelve inches (or 30.5 centimeters).

There’s so much information packed onto the Digi-Key PCB ruler that you might have a little trouble reading all of the information on it (not to mention your PCBs). Some of the print on the ruler’s sort of tiny. So are most of the SMT components, and the PCB pads and traces you might be comparing with the ruler.

Digi-Key has a solution for that too. I use a headband magnifier, like the inexpensive Aven 26225 to help me see tiny things. I’ve had a headband magnifier for more than 40 years, and I need to use it more and more each year. It must be the shrinking components; it surely can’t be my eyes.

About this author

Image of Steve Leibson Steve Leibson was a systems engineer for HP and Cadnetix, the Editor in Chief for EDN and Microprocessor Report, a tech blogger for Xilinx and Cadence (among others), and he served as the technology expert on two episodes of “The Next Wave with Leonard Nimoy.” He has helped design engineers develop better, faster, more reliable systems for 33 years.
More posts by Steve Leibson