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Guitar relies on printed electrodes, connectors

01 Mar 2011

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Paper Jamz is a toy electronic guitar that provides a surprisingly good approximation of an electric guitar, relying on three AAA batteries for power and its built-in 1.5-inch speaker for sound. Both its name and its low price of $25 hint that the guitar relies on printed-on electrodes and signal traces for its construction. It can serve as a case study in how sophisticated products can result from a powerful microcontroller and some capacitive-touch-sensing inputs.

At approximately 30-inch long, Paper Jamz is smaller than a typical electric guitar; considering its target audience, however, this size is understandable. Despite its name, it's not paper. Instead, it's a plastic, guitar-shaped shell that appears to be screen-printed with artwork including strings, a sound hole, frets, and volume knobs. It has three play modes. In freestyle mode, you refer to a chord chart that shows which frets to press for a chord. For major chords, pressing either one or two frets makes a chord. This use model is a far cry from using a real guitar, with which you must fret several strings at once. Paper Jamz is much simpler and allows young players to imitate rock musicians even if their fingers' dexterity isn't up to individual-string fretting.

1. You don't really need pressure to select a chord. With capacitive-touch sensing, just the presence of your finger is all it takes to select the chord of the mode or activate playing the strings. Capacitance exists between an electrode and any surrounding conductive material. The human body, although less conductive than a piece of copper wire, is still an adequate conductor. When a finger, for example, comes close to an electrode, the capacitance increases. A handy microcontroller senses that increase and serves as a triggering event. The sensing electrode is a grid of printed conductive traces that lie below the surface layer of plastic film.

A plastic-film tab with printed conductive traces wraps over from the back side and serves as a connector. The top of the neck cover secures the tab in place, clamping it onto the 1.5 × 2 inches PCB.

2. The whitish square on the far left side of the PCB is a flexible membrane that forms a cheap switch: The black dot is a bit of conductive material. Pressing it causes it to contact and complete a circuit between the PCB traces below it. It's accessible only before the manufacturer seals up the guitar, so it's probably a go/no-go test run to make sure the toy passes at least a simple end-of-assembly test. An eight-pin SOIC CE0030B chip from Chipower is a 1W, fully differential audio power amplifier with internal feedback resistors. The guts of the board seem to reside under a 1.59mm thick black blob of epoxy—too thin for a packaged part. This type of packaging, CoB (chip on board), or blob top, is likely the microcontroller. You can usually purchase less expensive chips as bare die and wire-bond them onto the PCB. Even allowing for the cost of wire bonding, it's still a cheaper manufacturing process.

3. The small yellow device on the power output from the AAA battery compartment is probably a PTC (positive-thermal-coefficient) over-current- and over-temperature-protection circuit to prevent short circuits and overheating of both the battery assembly and the circuit.

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