Turning a touchpad into a pen-iput device

March 5, 2005 2:29 PM

I've been racking my brain (and the internet) trying to figure out how to turn the touchpad on my new laptop into a defacto pen input device. Any ideas?

By now, it's more of a person project, but it would still be cool to be able to use it to jot notes and such...
The touchpad is on an IBM and is made by Synaptics. It is neither a touch/pressure/heat sensetive device. The only way I've been able to get it to respond to an input other than my finger is with a magnet, but it had to be laid flat on the surface. (It also seems to be only able to record one input at a time (so two fingers won't register) Thanks!

posted by stratastar to Computers & Internet (10 answers total)

I wonder if this is even possible. Unlike the writing area on a device like a Palm Pilot which is pressure-sensitive, touchpads like the one one on your laptop exploit Capacitance. You should read about how touchpads work.
posted by fourstar at 2:56 PM on March 5, 2005

I don't fully grasp the implimentation of capacitance, but I see no reason why you shouldn't be able to construct a faux finger, in the form of a pen.

Wikipedia brings up the point that touchpads offer relative motion, the touchpad isn't mapped to the screen area. That is apparently not true, see below.

I found this linux/X driver here, and one of their links there is a Synaptics "Touchpad interface guide" PDF. Section 2.3 outlines the absolute mode.

I think it would be more useful than you think.. tablets really don't have to be very large to be useful.
posted by Jack Karaoke at 3:43 PM on March 5, 2005

Surely you can't move the cursor on a touchpad without your finger on it? I suppose it depends what your pen input device is for, but if it's writing then the words will have to be joined together?
posted by fire&wings at 4:05 PM on March 5, 2005

Response by poster: Jack's mention of the "absolute mode" would make the joined-together together problem go away. The question then becomes about the tradeoff between the fixed space on the touchpad and the writing area on the screen...

I would be happy managing with cursive... (I could manage with a pad and a pen, but that I will propose is irrelevant.)
posted by stratastar at 5:23 PM on March 5, 2005

Wacom pads use a radio signal to track the pen movement. Your "clicks" come from a 2nd pen signal when a button is pressed (they have two buttons on them these days), or when a threshold of pressure is detected in the pad. In this case you'd have to toss out the radio portion, and just rely on pressure thresholds, which the pads already do.
posted by Jack Karaoke at 5:44 PM on March 5, 2005

Your "artificial finger" needs to be, have a blunt tip, since capacticance is directly proportional to surface area. Trackpads aren't sensitive to things with pen-like sharp tips. I've just tried an AAA battery (worked) and a screwdriver (didn't). I don't think you'd be able to write very well with something with a tip that large, the whole area of which needs to be pressed against the surface at all times.

Jack, trackpads aren't pressure sensitive, so I don't know how you'd click.

Here's a better aticle than the wkpd one.
posted by cillit bang at 6:12 PM on March 5, 2005

Response by poster: if you read the synaptics touchpad interface guide, the trackpads actually do measure pressure, (sortof) pressure is equated with contact area, so the more contact area (finger on pad) the more "pressure" is detected by the pad denoted by a Z vector, but it only does it in levels of concentric circles (I think). So it is possible to double click with the trackpad, and to click and drag, but it's more of a software translation of your action.

In either case, I feel like it would be possible to write an interface program to read and respond to different pressures as 'clicks', suitable for writing, but it seems pretty hard. I would suppose that the manufacturing companies would have done it by now if they thought it was possible.
posted by stratastar at 7:08 PM on March 5, 2005

Mine seems to require a minimum contact area of 5mm or so for the "pen" tip. Since it can reliably detect movements quite a bit smaller than that, this is probably a software-adjustable threshold that you could fix in the driver. If not, or if you aren't prepared to go about recompiling device drivers, you'll need something more like a crayon than a pen.

Also you'll want a soft pen tip, so that you don't have to hold it perfectly flat to get a good contact. Looking around, the most flexible solid conductor I see is aluminum foil, but that probably wouldn't work too well. You could also try soaking something in a liquid, but it would be tricky to keep it at just the right level of moisture (a sort of finger-like level, I would guess).

This is sounding like way more trouble than it's worth, but that just means you get more fame and fortune if you manage to find an easy way to do it.
posted by sfenders at 9:03 PM on March 5, 2005

Erk, yeah, they aren't pressure sensitive. That slipped out because I was thinking of Synaptics thresholds as pressure, when in fact that's just an easy end-user way to think of it. The capacitance thing still throws me for a loop, but we (humans) make all sorts of exotic materials these days, like conductive rubber and fabric.

I was going to say earlier that Synaptics is a pretty sharp company, and it seems like they would have exploited this were it feasible, but I didn't want to go raining on your parade. Maybe there's some reason other than $ they don't persue it.
posted by Jack Karaoke at 11:51 PM on March 5, 2005

I was intrigued by this picture and was wondering whether the electrodes are shown to scale. Fortunately I have an iBook in pieces handy and it looks like the spacing of the electrodes is 4mm. To track motion reliably, I'm guessing a finger must affect at least two x and two y electrodes at all times. A sharp tip won't do that, no matter what the sensitivity.

I think the conclusion is that without increasing the density of electrodes the only pen that's going to work is a conductive false rubber finger.
posted by cillit bang at 8:10 AM on March 6, 2005