Touch Mouse

Mouse operated by a touch switch


I created the Touch Mouse from a Logitec Marble Mouse USB, and have been using it successfully for about a year. I can now use the computer FAR longer without getting any pains in my arm.

It wasn't difficult, or terribly innovative, but thought I would share the design with others as it might help them too. Someone with a little experience of making electronic kits should have no problem making this.

The brass studs are simply brass split-pins available in stationers. The two studs on the 'button' cater for both right- and left-handed use, falling under the thumb and middle-finger respectively, and are simply wired to each other. (The left button on the mouse shell has been stuck in the 'up' position, so doesn't depress any more.) The other two studs are also connected to each other, form the other contact of the touch-switch, and naturally fall under the heel of the palm.

A Darlington Triple made from BC108 transistors amplifies the minute current which passes through someone's hand and also drives a miniature relay. You can hear it 'click' on and off when you touch and release one of the studs on the 'button'.

Moving the mouse pointer with the optically-read ball, and having a left-button operated by a touch-switch, has been my best solution yet to avoiding RSI (Repetitive Strain Injury).

There were a couple of obstacles to overcome:

  1. Mains Hum. Just touching the brass contacts on the button caused the relay to hum on and off at 50Hz (they are connected directly to the base of the first BC108). A large capacitor (1000uF) was connected across the output, and solved this. A small cap across the input gave the switch a time lag.
  2. Fitting it all in. A number of vero-board designs were drawn to pare it down to the space available in the mouse shell. Putting it all together was a little trickey, but well worth it!


Circuit Diagram

This was created using CircuitMaker STUDENT (the free download edition). The 0v and 5v are simply from the USB connecting wire. Most external USB sockets are overload protected, and I haven't fitted a fuse. If TP1 and TP2 are shorted with some wire, rather than a hand, then you risk over-loading and blowing a BC108.

Veroboard Layout

The ten squares represent breaks in the track. There are only two links, indicated by the thick black lines. Space on the board is very tight indeed.
Note the 'irregular' transistor pinouts. Carefully bend their legs to ensure you mount them correctly.
The large capacitor connects via leads so it can be positioned elsewhere in the mouse shell.
The microswitch which sat under the left button is removed and unsoldered from the Logitech board. Wires then connect from these points to SW1 and SW2. (ie: The switching contacts of the micro relay replace the microswitch.)

That's just about it! Any questions or comments please to: (Phil)

Useful Links

LOADS of vero board circuits, advice, free design software etc., especially Drawing the Veroboard Layouts which I used on this project.