Health and physical activity are intimately connected and the concern for health keeps on increasing. These make the measurement of physical activity an important issue. The purpose of this project was to develop a prototype of a physical activity monitor. The device can transmit the acquired data making a detailed study possible. The prototype based on a tri-axial accelerometer was successfully produced. It can be personalized by setting up the time and the recommended daily activity. The registered data can be uploaded via Bluetooth to mobile phones. The memory can store more than 18 hours with a sample rate of 1 Hz.