Capacitive Stylus for a Disposable Pen

I have a dozen Uni-ball Micro Roller Ball Pens. I want to add a capacitive stylus to the cap on one of them. Then the cap and stylus can be moved from one pen to the next to the next as each runs out of ink. I am grateful to Jason Poel Smith for his fine 2012 Instructable on different types of capacitive styli and ways to make them (DIY Capacitive Stylus—The link sometimes refuses to take me to the Instructable, itself.).

One way to make a stylus is to use the blunt negative (-) end of a AAA battery. I found that makes the top of a pen unnecessarily heavy. He also mentions using as much metal as practical with a lead to something metal touched by the user’s hand or fingers. He used a thumb tack. I am using a very short piece of steel rod.
The photo is my finished stylus. When this pen no longer writes, I will move the cap to an identical new pen. If the cap no longer works, I will mount the metal rod piece I made on a new cap.

3/8 inch steel rod

8-32 machine screw

Hot glue

My plan was to screw my metal piece to the plastic pen cap. I changed plans out of necessity and the screw stud in the cap is retained by hot glue, which is more forgiving than accurately placing and tapping threads in the cap.

Drill a hole about 5/16 inch deep into the end of a steel rod. It does not really need to be precisely centered, although I started with that in mind. But, the pen cap has a loose piece that complicates things. There will be more about that later. The drill I used is a numbered size for tapping 8-32 threads.

I cut the drilled portion of the rod, but long enough that I left the hole blind so the hole is closed and not open.

I taped 8-32 threads in the hole. Because the hole is blind, do not force the tap when it bottoms out.

I cut a longer 8-32 screw to not quite 3/8 inch in length. I used the bolt cutters on a common wire stripper tool. I did add a couple of nuts to the screw before cutting in case the threads were skewed and I could chase them by turning the nuts off.

I screwed the 8-32 stud into the hole I tapped. When drilling the hole I had hoped to thread I discovered the very top of the cap was a loose plug that came out while I was drilling a hole. There was a little space and a more solid recessed top deeper inside. I did not locate the hole through it well. One option was then to begin again with a new cap from another pen. I decided to enlarge the hole with a Dremel tool and anchor my screw stud in hot glue. See the second photo. Put the cap on the pen. (not shown in the photo) The hole is difficult to see, but I generously filled it with hot glue and quickly placed the screw stud in my metal stylus into the hot glue. Position it as best you can and hold it undisturbed until the glue cools and hardens. Try to position the steel rod piece so it touches the stainless steel pocket clip on the pen cap as much as possible. See the text box in the third photo. When using this as a stylus, let your fingers rest on the stainless steel pocket clip so the electrical capacitance in your body is in contact with the pen clip and flows to the piece of rod.
I ground the top end of my rod stylus to a slight dome shape. There were some machining marks on the end of the rod. I used a knife honing stone to smooth and polish the domed end of the rod and that improved the performance of my stylus. See the fourth photo. I found the stylus was not as sensitive as I hoped. An Ohmmeter told me the stainless steel clip and the piece of rod did not make electrical contact. I used a cold chisel to push a little steel toward the stainless steel pocket clip. Now there is electrical contact between the two, and the stylus is more sensitive.

For the sake of appearance I held steel wool over the metal stylus and twisted the pen to polish the metal a little.

My fingers work pretty well for most commands on an iPhone or an iPad. But, navigating Bible software on my Kindle can be difficult, even with a stylus. Spacing is tight between selections. My new stylus on a disposable pen is a big help. Practice will help to increase accuracy. Part of that practice is experimenting with the angle needed to get the best response. I find tapping on a chapter number works better if I aim for the underline below the number and favor the right corner of the number.

UPDATE: The Uniball pens I have were in a drawer for many years. Even if I add a few drops of water to the upper end of the cellulose ink reservoir, the pen dries out too soon. New pens might be better. Finally, I experimented with installing a Parker Gel refill. I grasped the roller ball point with a pliers and pulled it out. I used the same slip joint pliers to remove the plug at the top end of the pen body. I used a wire to push out the cellulose reservoir. Then I drilled out the end where the roller ball had been so the hole would fit the ball end of the Parker refill. I fitted a short wooden dowel on the top end of the Parker refill to fill the void between the top of the refill and the bottom of the plug. Parker ballpoint refills and Gel refills are the same size and either could be used.

And, I did another Instructable in which I replaced part of the dome on the pocket clip of a wooden European-style ball point pen with a wide head screw that makes a capacitive stylus. See it here.