Our particular unit, a Humminbird 597ci HD, was left over from another project and therefore a very cost-effective choice. While this unit retails for around $400 new, a quick check online reveals that there are many different fish finders compatible with this approach. The key requirements are a built-in GPS, and a NMEA0183 output. Any fish finder with both of those features should output the necessary data.
The data logger consists of a single circuit board that receives the NMEA0183 signal, translates it into a format that can be read by an onboard microcontroller, and then writes that data to a SD memory card in text format. A single LED illuminated button is used to activate the logger and provide feedback when recording points. Point acquisition is determined by straight-line distance from the last recorded point, but could alternately be based upon time, speed, surface temperature, or a combination of factors. For our work the microcontroller is programmed to record a point every five meters.
Both the data logger and its power source, two AA batteries, are housed in a small plastic enclosure and connected to the fish finder through a removable data cable (purchased from the manufacturer). This allows one to disconnect the logger from the fish finder when not actively engaged in mapping. While I haven’t extensively tested the battery life of the unit, calculations and field observations suggest that two alkaline AA batteries should be sufficient for well over 100 hours of mapping.
Designing and building a circuit board can seem intimidating but actually turns out to be a rather simple process. I used a CAD program to put together a schematic, which was then translated into a physical layout for the printed circuit board (PCB). From this one can generate manufacturing plots that can be uploaded to a manufacturer’s website. A few weeks later your boards arrive in the mail, generally in quantities of ten at a cost of about $2.50 a board. After that, all that is left is to solder on the components and program the microcontroller.