The coach of a volleyball team needs to predict when to substitue players before they get too exhausted. This will maximize their efficiency, due to the shorter recovery time. Fresher players would mean an advantage towards the opponents. By monitoring the height of the players’ jumps, their activity can be viewed. The system consists of two parts: a wireless sensor device and a smart phone. The device is mounted at the front-side of the pelvic, next to the hip bone. The jumps are detected by analysing the acceleration perpendicular to the ground. Whenever the device successfully detects a jump, the measured height will be transmitted via Bluetooth to the smart phone used by the coach. The smart phone platform uses Windows Mobile 6 Professional. A major part of this project was to design the wireless jump sensor device. The prototype consists of a development board with one microcontroller, an accelerometer, a Bluetooth module and a power supply which charges a Li-Ion battery and switches in the battery when it is not charging. Additional hardware was required for the power management. An application was written for the smart phone to communicate with the wireless device and graphically present the incoming measured heights. The algorithm to detect jumps and their measured heights was supplied from another internship, but required to be im- plemented for the wireless sensor device. Tests showed that the wireless device can successfully detect every jump and transmit the height to the smart phone. Analysis of the received heights showed that the resolution of the height measurement was less than 1centimeter. Nevertheless, the system would be too expensive to create in large scale, if the algorithm is not changed to one which is based on pattern recognition. Otherwise each device requires measurement of its accelerometer offset and compensation of it in the code.