Under water control data

[BrianT]

Hello Nick,
I had good success at up to 50 metres in fresh water with frequency shift keying. My transducers were smoke alarm piezo benders which were air backed and had a resonant frequency around 3000 Hz. Frequency separation was about 200 Hz. The transmitter driver was a MAX232 which gave +/- 8.5 volts of drive from a 5 volt rail. A PIC16F877 generated the two tones. A 4046 phase locked loop detected the two tones. Data rates were up to about 25 bps but typically under 5 bps depending on echoes, background noise, etc. Real world lakes gave much better range than hard walled pools. Hard wall swimming pools have standing waves and give strong signal or zero signal as the sensor is moved only a few hundred millimetres. Sea water has about 10 times the attenuation of fresh water. This is due to magnesium sulphate and boron salts. Add epsom salts and borax to a freshwater tank to simulate sea water. The sodium chloride appears to hardly matter and just causes corrosion.

HTH
BrianT