Wiring Pencil

Verowire and Road Runner pens/pencils are not available in Australia (at least as far as I've been able to find), so I've always built digital prototype circuits point to point with magnet wire by hand (ie juggling a spool of urethane enamel magnet wire, tweezers, side cutters and the soldering iron). Today I had to build a relatively simple interface board for LED matrix devices, but in frustration after dropping the spool and tangling the wire I finally got around to building a proper wiring pencil.

The Wiring Pencil

Some brass tubing from a hobby store bulk-pack was used. The pack came from Hobby Co and while moderately expensive contained a great variety of different sizes, all about 100 mm long. The smallest ID tube available was about 0.8 mm (allowing comfortable use of 125 um, 200 um and 500 um wire depending on the application), this was telescoped with several other pieces to build the barrel of the pencil. Some channel stock was used to attach the pencil barrel to a rectangle of PCB material which supports the sewing machine bobbin containing the wire. Electrical solder holds the works together - I had all the parts except the brass tubes sitting in the pile of junk of my desk. You could no doubt build something similar using a discarded writing pen, perhaps with a sports-ball inflation needle.

The Tip of the Wiring Pencil

Some felt was fixed to the PCB material to facilitate variable tension; the spool axle is a brass machine screw which is tapped into the thick PCB material, a cap nut is tightened onto the bolt and the entire bolt moved in or out of the PCB to control the friction between the bobbin and the felt. This works quite nicely to control the flow of wire.

The Spool of the Wiring Pencil

The bobbin was filled with wire by attaching it to my electric drill and running on enough wire to fill it from a larger spool. (BTW: a 6.5 mm audio jack just happens to be an excellent friction fit with the ~ 1/4" centre of the sewing machine bobbin. No doubt a bolt with washers would be a safer and more conventional choice - or perhaps a bobbin winder machine if you have access to one.)

The other side of the Wiring Pencil

I am yet to find a local source for something like the wire management combs both commercial prototyping systems use. It is said that the self-fluxing wire the commercial guys offer is easier to work with than common UCW - dunno, never used it. The UCW wire is quite easy to tin, especially the 125 um wire which tins even starting far from a cut end. I prefer the mechanical properties of 200 um wire for general purpose work, and use 500 um wire for lines carrying significant current, but I often put thicker lines in by hand anyway. You might like to have several spools (or better, complete pencils) of different colour wire, say red and green for power, amber for signal, etc.



2009-02-16: Solderability of Enamelled Copper Wire
Tests of tinning various types of enamelled copper wire both from cut ends and through unbroken enamel.