Sumlog interface

[alan43]

My boat has a VDO-Sumlog speed/log instrument with a hull-mounted propellor-type transducer, ie., NOT a paddlewheel.
Unlike the device described in a previous discussion thread, this propellor is not mechanically linked by cable all the way to the display unit, but has an interface just inside the hull, from which there is a 2-wire connection to the display unit.

This arrangement still works after a fashion, the speedo reading is at least plausible. However the log function does not work, and I would really like to replace the instrument with a more modern display. (The boat was built in 1978, & I think the instrument is original.)

Since the impeller still appears to be functional, I would like to continue to use it, rather than drill another hole in the hull to mount a paddlewheel. I have no reluctance to make up any necessary electronics, but I need some information first.

1 - Does anyone know about the sender unit for the Sumlog? Is it something as simple as a voltage-free contact closure (reed relay) for each 'pulse', or a solid-state switch, or is there any voltage or current actively generated by the device? Do I need to be concerned about supply or signal polarity? (So far no luck in getting this information from VDO.)

2 - Since I want to connect to an instrument which would normally have a paddlewheel input, then I need the same information about paddlewheel sensors. (Are all paddlewheels the same in these respects?)

3 - It would be nice to know the speed/frequency ratios for both the Sumlog and a typical paddlewheel.

Any assistance warmly appreciated.

 

[rogerm]

Since you have no other replies so far....
I am an electronics engineer who has dabbled with a number of these things over the years. My thoughts go like this: how many wires go to the impellor? if it is just two then the chances are that it is just a reed switch although it just might be a semicuductor device. The older it is the more likely it is a reed. If there are more than two wires then the likelyhood of a semiconductor device increases. See what voltages you can measure between the wires..... However you will likely be out of luck on the number of impulses for any given speed since impellors and paddle wheels tend to be quite a bit different on that score.
Roger

 

[HaraldS]

DOn't know what the sumlog of these days uses, but I know that Raymarin and Simrad and probably many more get their sensors from Airmar:

http://www.airmar.com/index1.html

Check out the specs on their site. Seems the common sensor has two magnets on a four blade paddle wheel, giving two pulses per rotation, which translates into about 10 pulses per knot.

I have seem three wire and five wire sensors. The five wire are with temperure sensor. The paddle wheel part will likely have three wires and works with a Hall-sensor. One is for common ground, one for the 12V supply and the third that delivers the puls is on an open collector output, which means it will pull to ground when the magnet comes by and float otherwise; it needs a pullup resistor.

I had digged into this a year ago to blend two sensors and feed the average to the instrument (Raytheon ST60). I had the problem that I got quite differing reading depending on which bow I sailed, at least when noticeably heeled.

So my little box drives the instrument with an open collector output. On the input side for the sensors I used Schmitt-Triggers, just to make sure I get a decicive signal.

By the way, the sensors seem to make a very balanced square wave with a one to one ratio on and off.