The Paper Jamz guitar :~ $25 or €27

Here’s what the Paper Jamz guitar looks like:

It’s smaller than the usual electric guitar, only about 30 inches long, but based on its target audience, this is understandable. And it’s not, at first glance, paper — It’s a plastic guitar-shaped shell that appears to be screen printed with artwork including strings, a sound hole, frets, and volume knobs.

How does it sound? Pretty darn good for a $25 guitar. It runs on 3 AAA batteries, and has its own 1½-inch  internal speaker that delivers a surprising amount of mediocre-fidelity sound. There are three play modes:  “perfect Play” mode 1 plays three oldie rock songs, even including vocals. So if you’re into air guitar, you have your choice of Sweet Home Alabama, Online, and My Maria.  Rhythm mode lays down a rhythm track and if you are able to brush your fingers across the printed strings over the printed sound hole of the guitar, the correct chord will play, whether or not your left hand is doing anything with fretting the printed strings over the printed frets on the guitar neck. “Freestyle” mode is the most guitar-like mode of all: You refer to a chord chart that shows which frets to press that correspond to a chord. For major chords, pressing either one or two frets makes a chord. This is a far cry from actual guitar fingering where you must press an individual string, usually several above several different frets at once. Paper Jamz is much simpler, and allows a very young player to imitate rock musicians even when his/or her finger dexterity isn’t up to individual string fretting.

I keep saying “You press the fret…” implying that pressure is needed to select the chord, but this isn’t quite true. Just the near presence of your finger is all it take to select the chord of the mode, or activate playing the strings, which implies capacitive touch sensing.  So what’s going on here? Is the plastic guitar shell just a huge printed circuit board? What lies beneath? Tear-down time!

Here’s shows the back of the toy:

The body is only about  1/8”-thick. The two knobs are there to attach the optional guitar strap; in the center is the batter compartment for the 3 AAA batteries. How does the guitar pick up the capacitive touch sensing? It turns out that this is where the printed electronics comes in…

Getting down to the capacitive electrodes was tricky. The surface art work is printed on a plastic film that lies on top of cardboard, that lies on top of another plastic film that has the actual circuit traces printed on it. Separating this sandwich of hard plastic guitar back/cardboard backing/printed circuit traces on plastic film/paper insulator/plastic film printed with surface artwork involved soaking the sandwich in the tub for about an hour:

Here’s the external plastic peeling back to show the printed electrode grid:

Here’s the film that’s printed with the capacitive touch sense electrodes separated from the guitar:

First, a brief description of how capacitive touch sensing works:  Capacitance exists between an electrode and any surrounding conductive material. The human body, while not as conductive as a nice piece of copper wire, is still a very adequate conductor. When a finger, for example, comes close to an electrode, the capacitance increases and can be detected by a handy microcontroller and serve as a triggering event.

The electrodes on the Paper Jamz are a square gird of printed traces and there is an electrode grid between each “fret”. (Each fret is merely a printed line, not a raised surface.)

The conductive traces from each electrode run down one side of the neck; There are ten frets, so there are ten lines that go from the base of the neck to the flexible connector traces forming the connection to the electronics pc board:

This picture was taken while the plastic film sandwich was still in place on the other side of the guitar; you can see the plastic traces serving as a connector wrapping over from the other side. There were originally secured in place with the top of the neck cover, clamping them onto the pc board.

There are a couple of interesting construction techniques on the pc board. The first is the flexible square plastic membrane on the far left side of the pc board that forms a very cheap switch: The black dot is a bit of conductive material that when pressed down contacts and completes a circuit between the pc board traces below it. It’s only accessible before the guitar is sealed up, so it’s probably a go/no-go test that’s run to make sure the toy passes at least a simple end-of assembly test. There is an 8-pin SOIC chip from a company called Chipower, a CE0030B, which is a 1W fully-differential audio power amplifier with internal feedback resistors, according to the Chipower website.  The brains of the board seems to reside under that thin black blob of epoxy which is only about 1/16-thick, not thick enough for a packaged part. What’s under there?

In talking with attendees last week at the MEMs Congress in Phoenix, I heard some possible explanations: Most likely the part, which I’m assuming is the microcontroller, can be purchased less expensively as a bare die, which means it must be wire-bonded onto the pc board. Even allowing for the cost of wire-bonding, it’s still a cheaper manufacturing process. This is a common manufacturing method in automobile industry and is called a blob-top.

Here’s the battery compartment that holds the 3 AAA batteries:

Notice the little yellow device that looks like a radial capacitor, but isn’t: It’s a positive thermal coefficient (PTC) over-current and over-temperature protection if there’s a short circuit. It prevents over-heating of both the battery assembly and the circuit in the toy in case of a fault in the toy’s electronics.   (Thanks to the pointer from David Gunderson, senior engineer at MicroPower, the battery pack design company.)

At some point I may take a dremel drill to the bob-top epoxy, but for now I have plumbed the depths of printed electronics in this year’s Christmas toys.

Posted by Margery Conner on November 8, 2010