Frankenmouse: Kensington Thinking Mouse with USB, scroll wheel, and optical sensor

While searching for a USB replacement for my beloved but obsolete Thinking Mouse, I tried Kensington's Mouse-in-a-box Optical Pro. This is a terrible mouse - too big for my hands, with flimsy, ill-positioned buttons - but the internal circuit board and switch positions are similar to those of the Thinking Mouse. With the aid of a soldering iron and liberal use of a Dremel tool, I built the Frankenmouse: the hardware of a Mouse-in-a-box Optical Pro transplanted into a Thinking Mouse case.

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Parts:

• One Kensington Thinking Mouse.
• One Kensington Mouse-in-a-box Optical Pro (USB) (part number 72112, not the later Mouse-in-a-box Optical Pro 2.0).

Tools:

• Soldering iron and solder
• Solder sucker
• Dremel tool (or other tools suitable for shaping plastic and cutting circuit boards)
• Small gauge wire with matching wire cutter and stripper
• Small Philips screwdrivers

0. Disclaimer

1. Dismantle

• The Thinking Mouse has two screws in the base under the sticker.
• Keep the shell of the Thinking Mouse. You may also want to keep the circuit board for later use (see below).
• The Optical Pro has four screws in the base, one under the foot in each corner.
• The Optical Pro's scroll wheel can be tricky to remove. I pulled the button side of the wheel axle up hard until it popped out of the blue base, and then slid the wheel out of the circuit board mount.
• Keep the Optical Pro's circuit board, USB cable, scroll wheel, blue base, and optical plate (the circular or rectangular piece of clear plastic). Marvel at the complexity of the molded plastic shell, then throw it away.

2. Rearrange switches

• The front two switches of the Optical Pro become the back two switches of the Thinking Mouse. They need to be flipped 180 degrees in order for the switch plunger to fall under the button. Unsolder a switch, turn it so the plunger is on the inside, resolder it, and add a wire connecting the outer two pins of that switch. Repeat for the other switch.
• Remove the switches for the Optical Pro's side buttons. They will be reinstalled as the Thinking Mouse's front two buttons when construction is almost complete.
• The switches for the Optical Pro are stiffer than those on the Thinking Mouse. You may want to replace the Optical Pro's switches with those from the remains of the Thinking Mouse.

3. Reshape the circuit board

• Some of these modifications will sever traces on the circuit board. Before cutting, remember to draw a picture of the existing traces so you can add replacement wires if necessary.
• The board needs to be shortened enough so the switches will fall under the Thinking Mouse's back two buttons. Examine the Thinking Mouse shell to determine the desired length.
• Target: the front corners of the board. Cut these as close as you can to the switches.
• Target: the sides of the front half of the board. Don't be too aggressive here; it's easy to widen the inside of the shell too.
• Target: the back of the board around the taillight. Watch for traces: one Optical Pro I used had traces behind the taillight, but another didn't. Fold the taillight straight up and cut close to the light's mount points. Leave the light folded up.
• Target: the edge between the two halves of the circuit board. Most of this area is wasted space, and we need these extra millimeters to shorten the board. Draw a picture of the traces connecting to the jumper wires on each side; cut the jumper wires; trim the boards back until you hit components; add wires to replace the jumper wires and any traces.
• After shaping the board and adding wires, you may want to test the mouse by plugging it into a computer. The taillight should light up, the two remaining button switches should work, and the cursor should move if you insert the optical plate and hold the mouse at the right height. Of course, you might fry your computer if you've reconstructed the circuit incorrectly.

4. Reshape the inside of the shell

• Remember the target position: the switches must land in the right place. Everything else merely needs to fit.
• Cut notches in the two screw posts so the back half of the board can slide in.
• The new circuit board will eventually rest at the same height as the old one, but some of the raised parts on the bottom need to be removed to make room for protruding pins.
• Cut notches in the side and back of the shell to make room for wide parts of the board.
• The back of the top half of the shell may need to be thinned to make room for the taillight. (This conveniently makes the plastic more translucent so the light will be more visible.)
• The scroll whell mount is tall enough to hit the bottom of one of the shell's button arms. Cut away part of the arm and make sure the button travels freely. Cut as little as possible; you don't want to weaken the buttons.
• When you're done, the board should fit snugly and two of the buttons should work.

5. Position optics

• The optical plate may be rectangular or circular, depending on the revision of your late Optical Pro mouse. If it's circular, cut down the sides to make it narrow enough to fit between the two screw posts.
• The right height for the optical plate is flush with the inside bottom of the shell. Flatten any protruding elements, and flatten or remove the part of the shell that rises at the old mouse ball ring.
• Make sure the mouse tracks well when you're done. If your circuit board is still too long, it may buckle slightly when the shell top is attached, or the back half of the board may ride up over the front half. Either of these may cause the optics to be at the wrong height.

6. Make room for the scroll wheel

• When dismantling the button assembly, watch out for two hooks at the very front of the two front buttons. These break off easily, though they won't be missed much if they do break.
• The thick cylinder of the scroll wheel that sits over the scroll wheel switch obstructs the button arm slightly, just like the scroll wheel mount on the other side. Thin the arm so the button and wheel move freely.
• I didn't reuse the scroll wheel attachment from the Optical Pro's blue base. This means the wheel is poorly supported and tends to slide from side to side. You may want to try to use that part of the base. If you instead allow the wheel to float, you may want to cut the scroll wheel hole slightly toward the scroll wheel mount, so the wheel can fit more securely in the mount.
• The wheel sits rather low and does not protrude much above the top of the mouse. Be generous with the opening on the top side so there's plenty of room for your finger to reach the wheel and turn it both ways. Also make sure you can comfortably click the wheel without pressing the other two buttons.
• Sharp edges on the outside around the wheel are obviously annoying.

7. Reattach switches

• The front switches need to be quite close to the back switches; notice how they're almost touching on the old Thinking Mouse. You may need to cut notches in the circuit board in order to mount the switches close enough. Watch out for a trace along the front of the board.
• Connect the two switches to the connection points for the old side switches. Leave the outside pins of each switch wired together.
• I found it convenient to cut off the front tip of the old Thinking Mouse circuit board to hold these switches in place. If you do this, there are two traps. The existing traces on the board fragment are wrong; sever them with the Dremel tool. There may be electrical contact between the main board and the new board fragment; I used a bit of masking tape to provide insulation, and another strip of tape to tie the two boards together.

8. Enjoy