When you run for exercise, you can often take your music with you; but if you are serious about how well you run and want to see the stats of your efforts, you are mostly out of luck. Now, inexpensive sensor technology and low-cost CMOS radios combine with specialized embedded microcontrollers to serve up a $30 wireless runner's gadget. The Nike+iPod Sport Kit links an in-shoe transmitter to an iPod Nano-connected receiver, so that all the facts and figures of a workout can be tracked and reviewed.

It seems likely that the product is focused as much on ancillary sales of footgear, Nano hardware and songs as it on on sales of the Sport Kit itself.

The kit relays activity from a shoe-based transmitter to a receiver dongle that plugs into the iPod Nano docking connector. (The unit is compatible, at present, with only the Nano versions of the iPod.)

A sensor, transmitting at 2.4 GHz, detects a runner's cadence, and a set of calibrating procedures makes a range of estimates--including time tracking, distance run, pace and calories burned--available in the iPod. If you want some real-time feedback, a spoken alert can indicate milestones, such as lap splits or mileage, during the course of a workout. Basically, the Nano now plays host to a virtual trainer. Once your run is done, you can plug in to download the results to iTunes, where you can review performance history. By synchronizing the data on a companion Nike Web site, runners can set objectives and monitor their progress.

The receiver measures 25 x 16 x 5.5 mm and weighs in at 3.4 grams, an 8 percent addition to the 43-gram weight of the Nano. The sensor/transmitter is 35 x 16 x 7.6 mm and 6.5 grams, perhaps a 2 percent addition to the weight of a typical running shoe. While Nike sells special shoes with an insole pocket for the transmitter, the device could conceivably be used with other footwear. But it may feel like a sizable pebble if not installed thoughtfully.

With a $30 price, low cost must be a key objective, so a minimalist component set is essential. Perhaps the most amazing finding was related to the sensing element used to detect runner action. Apple's FAQ states that "a sensitive piezoelectric accelerometer monitors your footstrike when you walk or run and determines the amount of time your foot spent on the ground. This contact time is directly related to your pace." In the transmitter implementation, this equates to a low-cost stroke of genius in the form of using a piezoelectric disk speaker--common to inexpensive toys and greeting cards--in reverse mode. By allowing some travel in the plate of the speaker, the piezoelectric effect causes the transducer to function as the pickup of foot action. About the size (and cost) of a dime, the speaker isn't driven to make sound, but rather monitored for motion-induced electrical signals.

Output from the piezo device goes into a Microchip PIC16F688 microcontroller (Data Sheet, Technical Paper). An on-board program memory and internal A/D converter allow it to function as the sole device for sensor signal processing and transmitter system control. Formatted output from the Microchip device goes to a Nordic Semiconductor RF transmitter (nRF2402G). By employing Gaussian frequency-shift-keying modulation, the part claims up to 100-kbit/second transmission, plenty for the needs of the Sport Kit transmitter. The RF power amplifier internal to the nRF2402G sends the signal to a stamped-metal strip antenna for transmission to the iPod-attached receiver.

Low-power operation is essential in the transmitter, since the unit's 3-volt lithium coin cell is not replaceable. An ultrasonically welded plastic case means that when you're out of juice, it's time to replace the shoe-borne portion of the Sport Kit. Not surprisingly, data sheets for the RF device and microcontroller make note of extraordinary power conservation attributes.

Pickup in the Nano-connected receiver is by way of a chip antenna. Signals are processed in a Nordic Semi nRF2401 transceiver chip. It integrates the radio subsystem in a single CMOS device, adding duplex communication capability with a two-channel receiver. Demodulated received signals are fed to a Texas Instruments MSP430F149 mixed-signal microcontroller, which takes care of all signal conditioning and wired interface to the host Nano. With the addition of three small-scale analog components, a few timing devices and some passives, the electronics for both transmitter and receiver are complete.

The $30 package probably costs less than $7 to manufacture, so profit margins are good, but it's on a small base. A once-every-few-years revenue of $20 or so is about the most that might be expected in follow-on Sport Kit hardware. Far bigger profits, however, lurk in the prospect of the twice-a-year (or more) $100-plus purchase of specialty running shoes. Similarly, the iPod ecosystem gets a boost with possible player and iTunes sales.

In the end, shoes, songs, upgrades and brand buzz may be more the objective than big profits from the Sport Kit itself.

By David Carey, president of Portelligent (Austin, Texas), which produces teardown reports and related industry research on wireless, mobile and personal electronics (www.teardown.com)