My sister is left-handed and she shares a computer (and the mouse) with her husband who's right-handed. There are so-called ambidextrous mice, but they are just symmetrical and don't have the back and forward buttons on the side. Switching between left- and right-handed operation has to be done by changing the mouse settings in the operating system. I haven't found any mouse that can switch on a hardware level.

This gave me the idea to modify a mouse to allow for hardware level switching between left- and right-handed operation.

This is a pretty simple project that takes about an hour to finish. It does however involve soldering of tiny connections, what can be tricky for inexperienced solderers (like me).

Materials needed:

1 mouse
1 dpdt sliding switch (double on-on switch)
some electric wire
solder
hot glue (or regular glue)

Tools needed:

soldering iron
screwdriver
dremel (or saw, knife, file)
hot glue gun (optional)
scissors and wire cutter (or any other tools for cutting and stripping wire)

Step 1: Get a Mouse

If you don't already have a mouse that you would like to risk ruining, go get a mouse. I bought mine off AliExpress for $5,20, so it wouldn't give me a headache if the modification would fail. When choosing a mouse it's wise to take a large one, so it's easier to fit the button in it. Also make sure it's symmetrical and doesn't have buttons on the side.

Step 2: Open Up the Mouse and Make a Plan

Before opening up the mouse check if it's working, you never know. If the mouse is working alright, open it up. Usually there are some screws hidden under plastic pads that are stuck to the bottom of the mouse. Remove the pads ad unscrew the screws.

When the mouse is opened, decide where would be the best place to put the switch. Mine has a lot of space in the back part, so there it will go. Smaller mice usually don't have this luxury of space, so you could put the switch in the hole that is used to store the wireless receiver (if any).

Step 3: Make a Hole for the Switch

Cut a hole for the switch. I did it with a dremel, but you could also do it with a saw, knife, file,...

Step 4: Adjust Mouse PCB

Take the PCB out of the mouse and locate the leads for the left and right button. They should be easy to find; each button has two leads. One is ground you can recognize it by the fact that it's shared by the left and right button, we won't touch that lead. The other lead should be the +, each button has one that ends up to a pin of a chip on the PCB. For clarity I have marked those leads with a marker. Cut those leads somewhere where you have good access to them, using a dremel or a file.

Step 5: Prepare the Switch

The outmost pins of the switch should be cross connected (see illustration) and two wires will later be connected to the buttons. Two wires from the middle pins will be connected to the chip on the PCB. I put a piece of plastic between the two rows of pins to avoid short circuits. Keep the wires that will be soldered to the PCB long enough! I had to resolder one wire, because it was to short.

Step 6: Solder Wires to PCB

Solder the wires to the PCB. First solder the wires that come from the outer pins of the switch to the pins of the buttons. It doesn't matter which wire goes to which button.

After that solder the wires that come from the middle pins of the switch to the chip. Mind the colors of the wires, matching colors should be on the same (cut through) lead. I was so lucky that the chip was on a separate board, so I could solder the wires to the connectors of that board. I've also seen mice where the leads go directly into the chip (black blob). In that case you should clean a piece of the lead, so the copper is bare and solder the wire onto that.

As always when soldering, check for short-circuits and fix it if you have any. For small short circuits I usually just file the excess solder away. For bigger ones it could be necessary to desolder the joint and resolder it.

Step 7: Put It Back Together

First put the PCB back into the mouse and connect the power wire (if any). Rough up the plastic on the inside of the mouse body on the spot where the button will be placed (scratching with a screw driver). After that use a royal amount of hot glue to fix the switch in its place. My switch had holes in the side, so I closed them with tape, so the glue wouldn't get in and get it stuck. The wires going to the buttons should be on the right side, so the mouse will function right-handed when the slider is to the right and left-handed when the slider is to the left.

Arrange your wires so they aren't in the way of any constructional parts on the top part of the mouse, put on the top, refasten the screws and put back the sticky plastic pads.

Test your mouse. If you didn't connect any wires the wrong way, you should now have a true ambidextrous mouse.