ianj99
Well-Known Member
I've done this to interface a standard 5Hz per knot speed transducer to my nmea 0183 gear. My 30year old Seafarer speed log has not worked for years despite the impeller being free to spin so a cheap alternative type was recently won on ebay for under £20 which probably won't be Seafarer compatible.
The interface is very simple to make and I can supply a preprogrammed microcontroller for a few pounds for anyone who would like to do the same. Current consumption is a few ma.
Its been tested with a square wave so far and the NMEA sentence VHW is used as you can see from the Navmonpc display. The sentence header and id can be changed to anything - SDVHW might be more appropriate than IIVHW.
The same hardware could also be used to convert engine rpm, or any other varying frequency pulse. I will be adding a means of calibration next.
The screen shots show the Nmea data in the Navmonpc's monitor window and also as STW in the left hand pane; the scope window shows the frequency in the right hand pane.
The next refinement is to add a NMEA input so incoming data can be merged (multiplexed) with the speed data for situations where a spare nmea input is not available.
Ian
The interface is very simple to make and I can supply a preprogrammed microcontroller for a few pounds for anyone who would like to do the same. Current consumption is a few ma.
Its been tested with a square wave so far and the NMEA sentence VHW is used as you can see from the Navmonpc display. The sentence header and id can be changed to anything - SDVHW might be more appropriate than IIVHW.
The same hardware could also be used to convert engine rpm, or any other varying frequency pulse. I will be adding a means of calibration next.
The screen shots show the Nmea data in the Navmonpc's monitor window and also as STW in the left hand pane; the scope window shows the frequency in the right hand pane.
The next refinement is to add a NMEA input so incoming data can be merged (multiplexed) with the speed data for situations where a spare nmea input is not available.
Ian
