Sometimes, our simplest gadgets provide a world of convenience. For those who love to keep their cars running right"and I admit to being a fanatic here"tire pressure is a critical element of performance and more practically, good gas mileage. Having suffered through scores of error-prone mechanical tire gauges over the years, a $10 investment at Wal-Mart brought me into the modern age with the purchase of a Victor digital tire pressure monitor. Boasting a 3-digit LCD readout and 0.5PSI accuracy, Victor's "Digital Tire Gauge" designation fails to convey the fact that analog electronics are at the heart of the product.
To get from valve stem to PSI readout, a MEMs device from Measurement Systems Inc.'s IC Sensors Division San Jose, CA) translates air pressure to a measurable analog level. An air tube from the measuring head delivers tire pressure into an O-ring sealed enclosure screwed to the circuit board around the IC Sensors MEMs chip. The IC Sensors part (#331a) is a microfabricated piezoresistive bridge network built on the silicon surface, with the chip having a back-etched chamber that leaves membrane surface features behind. The piezoresistive membrane elements deflect with changes inside the pressure chamber, and the bridge elements' displacement leads to a measurable resistance change for downstream translation to a specific tire pressure. Interestingly the MEMS chip is wirebonded directly to the small circuit board and overcoated with a protective layer of silicone, apparently all without interfering in the transfer of air pressure to the underlying MEMs chip surface.
Sensor aside, there is still much analog work to be done. A custom chip from AMI Semiconductor (Pocatello, ID.) is the mixed signal workhorse of the little tire gauge, handling everything from the MEMs sensor interface to the segment LCD driver function. The AMI part is a custom device, thus exact functions remain a bit hazy. In support of the MEMs part, it must generate the reference voltage and monitor the output voltage of the bridge, the latter varying in proportion to resistive changes of the IC Sensor chip. The sensed voltage from the bridge is then presumably put through an analog-to-digital converter where a lookup scheme is likely used to translate digital values into a specific PSI reading. Tire pressure is then displayed on the 3-digit segment LCD connected directly to and driven by the AMI part.
Remaining electronics are a handful of passive components and a simple switch"actuated in the pressure chamber"to turn the unit on for a reading. An auto-timer in the AMI chip turns power off after about a minute to conserve the internal CR2032 coin cell Lithium battery. While not a headline-stealing product, the tire gauge is a great widget enabled by mixed-signal components. Once governmental mandates for automatic tire pressure monitoring in new cars come into force, similar devices could become very big business.
David Carey is President of Portelligent. The Austin, Texas company produces teardown reports and related industry research on Wireless, Mobile, and Personal Electronics. (www.teardown.com)