stefan klocek

USB Powered Magic Trackpad

September 28, 2014

I’ve done a number of replace-batteries with a charger projects (biggest savings was for a child-swing). Having recently purchased an Apple Magic Trackpad I disliked that it required batteries. It makes some sense for a mouse to loose the cord, but a trackpad is stationary and corded (aesthetics aside) power simply makes one less device that I need to recharge.

untitled (2 of 64) — Original battery powered trackpad.

The trackpad is nor — The trackpad is powered by a couple of AA batteries or 3 volts.

untitled (8 of 64) — I took an old USB cord and cut off the female connector.

untitled (13 of 64) — I started with a USB 2.0 cord so I had four wires after removing the insulation. Because you’re just adding power you don’t need the ground or the green wire (which is for data), just the red and black.

untitled (14 of 64) — I used a 1/2 dowel as a mass replacement for the two AA batteries.

untitled (17 of 64) — The fit is slightly loose, I tightened it up by adding a few wraps of electrical tape to both ends.

untitled (20 of 64) — I wanted a really clean replacement with all the guts on the inside. I bored out the center of the dowel to thread the power through it.

untitled (24 of 64) — Electrical tape increasing the width of the dowel.

untitled (25 of 64) — The far end of the battery compartment has a slight spring loading, but to improve the fit I bored out a little wider hole on each end in which I could place a spring.

untitled (30 of 64) — Next I needed an exit for the power. I could have bored into the wall of the battery container but it would have made insertion harder. I opted to drill out screw-top lid.

untitled (33 of 64) — To keep the lid from spinning as a drilled it out, I placed a washer in my vice and set the lid slot over it.

untitled (39 of 64) — Wire threaded through the lid.

untitled (55 of 64) — Fully assembled battery replacement.

untitled (45 of 64) — I tried the direct power from the USB (5 volts) and it worked. But given that that’s almost twice the original power I decided to add a resistor to bring it closer to 3 volts.

untitled (57 of 64) — I soldered in a 4.7 mega ohm resistor to the positive end spring, bringing down the power to ~3.4 volts, close enough. After sharing this design I got feedback from some smart people who told me I didn’t need to worry about the resistor and in fact the one I had chosen was likely to make the pad perform unreliably. I subsequently removed the resistor and have had a solid performance from the trackpad.

untitled (51 of 64) — Finished modification, almost neat enough for Apple.