The computer mouse was one of several remarkable innovations created by Douglas Engelbart and his team at SRI back in the 1960s. Although nearly everyone is familiar with the mouse, which is typically used by the dominant hand, few people are familiar with the input device they developed for the non-dominant hand. It is called a keyset (or sometimes, a chord keyboard).

For details about the history of the device, Ken Shirriff has an excellent blog post.

I made some measurements of the keyset Ken had on loan and 3D-printed a replica. With these files, you can print one for yourself as well!

Note: Instead of the lever-less 1SM2, consider using a switch with a lever like the 311SM1-T or the E-Switch MS0850503F040S1A

I used an Ender 3 with a 0.4mm nozzle and white PLA filament for the 3D-printed parts. A textured bed is optional but you will get a nicer finish on the keys this way. The keys may warp after printing, but you can correct this with a hair dryer and a little force.

For an extra nice touch, consider painting the keys and the top cover. The usual approach is to use a filling primer, then sand the part, then paint again, repeating until the "lines" disappear. Then apply a final coat. Use an off-white paint to match the original color.

You can tap the screw holes on the parts to make it easier to thread the screws through them, or you can just thread the machines screws directly into the part.

• Keys: two 4-40
• Pillow blocks: two 2-56, two 4-40, one 6-32

The base plate is made of a piece of 16-gauge sheet steel. I like to use a sheet metal hand punch to make the holes, but you can drill them if you like. Use a center punch to prevent your drill bit from wandering. A center punch divot is helpful to locate the hand punch die if you opt for that approach.

The base plate should be primed and painted as well.

A piece of adhesive-backed cork should be attached to the base plate after the paint has dried. The holes in the cork can be made with a sheet metal hand punch, but a paper hole punch should give good results as well.

Don't forget to cut out the 5 squares of cork used to cushion the keys. The cork squares should be mounted about 1.5" from the ends of the keys on the underside. Do this before installing the keys in the keyset.

The pivot rod can be cut to length with a rotary tool or hacksaw. Ideally you should use it with retaining rings which require grooves to be cut. You could do this with a lathe, or by putting it in the chuck of a drill press and using a file to cut the grooves. For mine, I press-fit two split-ring lock washers instead.

It's easiest to attach the switches to the pillow block pieces before mounting them to the base plate. When viewed from the top, the leftmost pillow block has a switch mounted on the left side. The 2nd-leftmost pillow block is the special one with the trimmed corner to allow for the cable to pass through the hole in the back of the cover. It also has a switch mounted on the left side.

The 2nd-to-rightmost pillow block has two switches mounted on it, one on each side. The rightmost pillow block has a switch mounted on the right.

All switches should be positioned so the actuator button is towards the rear of the keyset. The actuator button must line up with the rearmost hole in the top of the key. Eventually there will be an adjustment screw here that will push the actuator.

As an alternative, you might also consider using a microswitch that has a built-in lever. This should be installed "backwards" from the normal switch that has no lever. An advantage of this switch is that it requires less actuation force due to the leverage and might be more comfortable for you to use.

Now use 10x 4-40 screws to mount the pillow blocks to the base plate. Leave them a little loose so you can insert the pivot rod in the next step.

Place the keys at the ends of and in between the pillow blocks so that there are five in a row. The flat, textured side should be facing up. Install a retaining ring at one end of the pivot ride. Slide the pivot rod in one end, through all the pillow block and key pivot holes, out the other end, and secure it with the other retaining ring. The keys should be free to move up and down. Tighten down all ten screws that hold the pillow blocks in place.

Install 5x 4-40 screws on the top of the keys at the hole closest to the pivot point. These screws adjust the position of the keys in their normal at-rest position. Flip the keyset over and install 5x 4-40 screws into the keys through the access holes in the base plate. These set the trigger points of the switches.

To adjust the trigger point screws, push the key down and then tighten the screw until you hear the switch click. Release the key and make sure the switch disengages (making another click).

To adjust the rest-point screws, tighten the screw until you hear the switch click, and then back it out a little until the switch clicks again. You also want to make sure that all the keys line up visually while at rest.

The original keyset uses an unusual multi-conductor cable with no jacket. It has six grey 26-gauge wires arranged into twisted pairs that are then twisted together. One wire goes to the common connection of all the switches (the rear terminal) while the remaining five wires are connected one to each switch (center terminal).

There are two pieces of heat shrink tubing. One nestles in the 1/4" holes in the pillow blocks and helps trap the cable in place. The other goes between the cable and the hole in the top cover to act as a strain relief.

Run the cable through the hole in the top cover and pull it all the way through. The top cover should slip over the pillow blocks and sit flush with the metal base plate. Install the two flat head screws to hold it in place.

If you want to connect your keyswitch to a parallel port, the wiring diagram is below. There is a 1.5K resistor (optional) used for presence detect. Otherwise, just run the wires to your Teensy board. Ken Shirriff's blog entry has more information.