19 RayMarine Signals
RayMarine doesn’t provide ordinary consumers with data on the internal workings of their instruments, so we need to put our oscilloscope on the instrument leads to find out what is going on. By measuring how they perform, we can make some guesses about what’s inside.
The signals from the wind, speed, and temperature sensors are slow enough and simple enough that we could process them in software on a microcontroller without needing too much expertise. RayMarine has already looked after that for us in this case, but we could use what we have learned to point us in the right direction for measurements of other quantities.
AirMar Speed / Temperature / Depth
The system combines all three of these functions in a single through hull transducer sourced from AirMar with a paddle wheel on one side and piezo echo sounder on the other side.
Depth
Shield is ground
Blue shows bursts, approaching 500 volts peak to peak at the start of each burst and sustaining at about 120 volts peak to peak. The period is about 5 microseconds for a frequency of 200 kHz, which matches up with the specs on the analog version of the AirMar DST800. Bursts come about 3 times per second, with each burst lasting about 650 microseconds.
Black shows the same bursts, but at a lower voltage. Max is about 140 V peak to peak, settling down to about 30 V peak to peak.
Neither seems to be affected by moving the transducer. They are 180 degrees out of phase with each other, so the peak difference between the signals is about 640 volts.
With the speed of sound in water at about 1500 m/s, the return signal would lag the sent signal by about 1.33 ms per metre of water depth for a two way round trip. In air, there is no return signal evident on the scope unless it is hiding in the noise. This is probably better left to electrical engineers. Those 200 kHz signals are faster than we can easily handle in software and will need some analog expertise.
Speed
Red is the 8 volt power supply.
Green returns a voltage of either about 0.2 volts, or about 7.5 volts, depending on the position of the paddlewheel. A full turn of the wheel yields hi-lo-hi-low for the four bladed wheel, suggesting a small magnet in each of two opposing paddles for weight balance, sensed by a Hall effect sensor or similar in the body of the transducer. A simple digital pulse train is easy to process, especially at relatively low frequencies like the 5.6 Hz / knot listed in the spec sheet.
Temperature
White shows about 0.78 volts at room temperature and Brown shows close to zero at room temperature. They are probably the two leads to a thermistor for temperature measurement.
Wind
Red and Black are 8 volts and ground to power the anemometer.
Wind Speed
Yellow shows pulses with two regions of higher voltage per rotation, but the pulses are not nice even on/off switching. The analog level output stays fixed for a given angular position, even if the anemometer is not turning.
Wind Direction
Green and Blue are conti