Pedometers, now popular as an everyday exercise progress monitor and motivator, can encourage individuals to compete with themselves in getting fit and losing weight. Early designs used a weighted mechanical switch to detect steps, plus a simple counter. When these devices are shaken, one can hear a metal ball sliding back and forth, or a pendulum striking stops as it swings.

Today, advanced pedometers rely on microelectromechanical systems (MEMS) inertial sensors and sophisticated software to detect true steps with high probability; MEMS inertial sensors permit more accurate detection of steps and fewer false positives. Taking advantage of the low cost and minimal space- and power requirements of MEMS inertial sensors, pedometers are being integrated into an increasing number of portable consumer electronic devices—such as music players and mobile phones. The small, thin, low-power ADXL335, ADXL345, and ADXL346 3-axis accelerometers from Analog Devices are very suitable for such applications.

This article, based on a study of the characteristics of each step a person takes, describes a reference design using the 3-axis ADXL345 accelerometer in a full-featured pedometer that can recognize and count steps, as well as measure distance, speed, and—to an extent—calories burned.

The ADXL345’s proprietary (patent pending), on-chip, 32-level first-in, first-out (FIFO) buffer can store data and operate on it for pedometer applications to minimize host processor intervention, thus saving system power—a big concern for portable devices. Its 13-bit resolution (4 mg/LSB) allows pedometers to even measure low-speed walking (where each step represents about 55 mg of acceleration change) with reasonable accuracy.