Fish Finder Depth Sensor

Sadly we did not win one of the five Volta Cohort investments last week, but it was still a valuable experience nonetheless. We were able to practice refining our pitch and improve the focus of the business, which should help us going forward.

The next couple of days were spent writing driver software and hooking up cables to convert the RS-232 output from the fish finder into depth measurements for AMOS's Raspberry Pi. I bought a 3 foot long RS-232 to USB cable for this purpose, but couldn't find the male DB-9 plug for soldering that I thought I had lying around somewhere. So  I snipped the cable near the end and pulled out the bare circuit board inside:

This afforded some better soldering points for the white, green, and black wires on the top surface of the board. And as luck would have it, I didn't mix up the green and white TX/RX cables.

The fish finder outputs NMEA 0183 sentences in ASCII text at 4800 bps, something like this:

$INDPT,1.1,0.0*47
$INHDG,,,V,,V*60
$INHDT,,T*0B
$INGLL,,,,,,V*16
$INVTG,,T,,M,,N,,K*5E
$INMTW,10.1,C*14
$PSMT,0,0,0,2,appver,0*28
$INDPT,1.1,0.0*47
$INHDG,,,V,,V*60
$INHDT,,T*0B
$INRMC,,V,,,,,,,,,*21
$PSMT,0,0,0,2,appver,0*28
$INDPT,1.1,0.0*47
$INHDG,,,V,,V*60
$INHDT,,T*0B

The lines that matter are the ones that start with either $INDPT or $INMTW. These correspond to either depth or temperature respectively. Fortunately there are lots of websites around that summarize what the NMEA sentences mean, so I didn't need to pay $1000 or more for the official standard. Writing the software driver for the Pi to parse this info was pretty straightforward. Just needed to make sure that the software polled the serial port frequently enough to keep the depth data current. Depth is sent out only once per second by the device, so it's not exactly a compute-intensive task.

On Sunday Kirsten and I took advantage of some good weather to take the canoe and AMOS out for a test. Unfortunately AMOS kept veering off to the left for no known reason. A later inspection back at the house indicated that one of the electronic compass wires had come unplugged. The following day I tried again, but the same thing happened, it kept trying to go west when it should have been going north. Wanting to test the fish finder anyway, I towed AMOS around in the kayak for about an hour. I didn't cover quite as much water as I thought I did, but the results for the depth measurements were pretty good:

and with interpolation turned on:

That evening I checked the compass calibration on the grass in our backward and found that it was indeed off by about 90 degrees, i.e. when the boat was pointed north the compass output indicated that it was facing east. I'm not sure yet if this is some new hardware problem, or if the compass calibration got messed up somehow and needs to be re-calibrated. Hopefully that'll get solved tomorrow.