Low-cost short -range wireless optical FSK modem for swimmers feedback

Abstract

This paper reports 3 axis accelerometer data transfer over a one meter underwater path at 10 cm depth using a 2400 bps optical wireless frequency shift keying (FSK) at very low frequency (VLF). The modulation frequencies used were 10 and 12 KHz. The prototype modem was designed and implemented for real time feedback for swimmers in the pool. The optical transmitter included an accelerometer unit with a microcontroller, the modulator and a detector circuit based on an integrated detector preamplifier (IDP). The cost of the components for the optical transmitter and receiver was less than AU$25. Range experiments were performed in air and underwater, with and without bubbles. The received data was error free for 1.3 m in air and for more than 1.1 m underwater without bubbles. The underwater range decreased to 70 cm with bubbles. The availability of the link between the wrist and head of a swimmer was approximately 50% and varied with the position of the wrist. This enables stroke rate data to be presented to the swimmer via a goggle mounted display.