Fishfinder temperature resistance mod

Discussion in 'General Electronics Chat' started by sdowney717, Jun 15, 2015.

  1. sdowney717

    Thread Starter Active Member

    Jul 18, 2012
    I have an Eagle 320c which I modified to use an Airmar 7 pin plug transducer.
    The depth sounding works fine.
    The temperature reads 20* too low.

    the original transducer, the temperature resistor had failed, reads about a million ohms and Eagle 320c was saying it was minus 10*F.
    With the Airmar 7 pin transducer, the Eagle 320c reports a 77* true temperature as 55*.
    So my theory is the lower the ohms, the higher the temperature readout on the display.
    I measured the Airmar 7 pin transducer temp wires and they read about 8300ohms at 77* true water temp.
    So my idea is place a resistor in parallel with the temp sensor in the transducer, that will lower the ohms and bring up the reading.

    Question is how big ohms resistor to use?
    Preferably, an adjustable one would be ideal. What range should I look at getting?
    I was thinking 0 to 100,000 ohms range pot, if that exists.

    here is the link to schematic for the Airmar 7 pin

    here is my mod, I paralleled the wires inside the 320c to use the plugs for a 7 pin setup.
    So I can either use the original slightly broken transducer, or the Airmar 7 pin transducer.
  2. crutschow


    Mar 14, 2008
    You could try a pot in parallel (100kΩ is probably a good value to start) but note that even if you adjust the pot to give the correct reading at one temperature, the other temperatures may still give readings that are off. You will need to check that over the temperature range you need.
  3. #12


    Nov 30, 2010
    Your primary problem is that thermistors are not linear. If you're smart and lucky, you can figure out which thermistor is the right one and attach it to the wires. They are usually rated as, "X ohms at 25C". Find the resistance that registers as 25C and you might buy the right one.
  4. debe

    Senior Member

    Sep 21, 2010
    Heres some pics how to determine the orig value of the NTC thermistor. Use a variable resistor & connect across the appropriate conectors & power up sounder. Adjust the resistor until you have 25degC, remove var res & measure the value. This Eagle sounder was 5K ohm for 25degC. Ive found most readily available NTC thermistors ive purchased have been accurate enough. You will find most Eagle sounders use the same NTC value. First picture was the amb temp here from the transducer assembly. AMB TEMP.JPG WIRING.1.JPG RESISTANCE 25 degC.JPG SIM NTC res.1.JPG
  5. sdowney717

    Thread Starter Active Member

    Jul 18, 2012
    Updating, it is working my mod.
    I used a 100k minipot.
    However I had to adjust to the far end, so all it needs is a few thousand ohms to dial in the temp reading.
    So for the other one, I will use one of the 50k minipots. I had bought 50k, 100k, 200k off ebay for a few dollars, I got 15 of them total.

    I checked the water temp with my meter first, then dialed in the temp reading on the sounder to match.

    How much watts can these blue minipots take?

    Here it is hot glued to board in a very convenient perfect spot, fits great.
    Green wire is ground, yellow wire is temp1 coming from transducer.

    showing it reading 82*F

    The FishEasy also can adjust using the builtin software the speed wheel calibration. It needed a 50% boost, so I have all 3 functions working, depth, speed, temp using an Airmar bronze thruhull transducer .
    debe likes this.