Personal health steps into a seldom-seen spotlight in this space as we examine the Omron IntelliSense HEM-711AC blood pressure monitor. The unit accomplishes some sophisticated metrology for a surprisingly low price, and comes in a package that's usable at home or on the go. With a retail price of around $60, the product clearly relies on low-cost design, but the need for accuracy also requires some clever engineering.

A division of the diversified Kyoto, Japan-based Omron, Omron Healthcare Inc. (OHI) is likely the top maker of blood pressure monitors for personal use, and the group's focus is firmly rooted in health-monitoring products for both consumers and medical professionals.

The HEM-711AC comes with the familiar arm cuff and a base unit roughly the size of a small paperback. While not tiny enough to fit in a pocket, the device is certainly portable, and a push of a button is all it takes to make the system inflate the arm cuff to the appropriate pressure. Electronics then automatically take measurements for systolic and diastolic pressure, along with heart rate, and display the data on a 2.5-inch liquid-crystal display. The unit is powered by four AA alkaline cells or a supplied ac adapter.

The HEM-711AC-along with most other personal blood pressure monitors sold-uses the "oscillometric" technique for measurement. Oscillometric monitoring relies on the tracking of pressure oscillation patterns in the cuff during its stepwise deflation. Proprietary algorithms then translate observed fluctuations into systolic and diastolic pressure values.

While there has been much debate about its usefulness for doctors, the general consensus seems to be that the technique is good enough for general trend monitoring.

Internally, the HEM-711AC contains a motor-pump combination, an electronically operated air pressure release solenoid valve, the 2.5-inch LCD, a single circuit board and air tubing to connect the pressurized components.

The rubber tube from the cuff enters the base unit and is plugged into a pressure sensor mounted directly on the circuit board. The same rubber hose is connected to the pump and release valve, which handle the pneumatics. The pressure sensor in this case is a mechanical diaphragm that is depressed in proportion to applied pressure. The deflected "drumhead" of the diaphragm forms one of two plates of what is effectively an air-dielectric capacitor, with a fixed plate in the pressure sensor parallel to the movable plate. Pressure changes in the system now correspond to shifts in the variable capacitor.

The clever part is turning capacitance changes into useful data on the cheap. The precise details of implementation get a little fuzzy at this point, but trace-out of the circuit suggests that the variable capacitor (pressure sensor) is used to modify the frequency of a ring oscillator formed from the four gates in the Toshiba 74VHC02 quad NOR gate. Wiggles in pressure now correspond to wiggles in the ring-oscillator frequency. Its output is connected to a proprietary 8-bit microcontroller from Toshiba, the CK29U-3B53, which acts as the unit's electronic workhorse.

Exactly what happens next in the microcontroller is also a little unclear, but some form of frequency counter or frequency-to-voltage conversion is presumed. The output of either is used to drive the internal algorithms, which determine a corresponding systolic or diastolic pressure value for display on the LCD. The Toshiba microcontroller drives the LCD directly. It also controls the buttons the user pushes, the pump motor and air pressure release solenoid. A Seiko Instruments E2PROM, the S-29221, holds the limited amount of boot code needed.

Detailed analysis of the product pegs manufacturing cost-of-goods-sold below $20, with a roughly even split between electronics and system mechanicals.

The simple elegance of the transducer/ring oscillator solution at the heart of the HEM-711AC is critical to the low observed COGS. By the addition of simple, inexpensive electronics around the sensing method, an important health-monitoring tool can be delivered to a mass consumer base at a nice profit.

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