Hello -- I have sailboat with an (existing) speed-sensing ‘paddlewheel’ sensor that I am trying to use with a new data display. I have gotten lucky because the sensor and the data display both have the same frequency per knot (~6Hz/knot) – however the existing sensor generates an AC signal whose frequency is proportional to the speed over water (similar to a variable reluctor pickup in a car, I suspect), while the data display is of the ‘hall effect’ style and expects pulses of approximately 11.2V DC in amplitude.
In other words, I need to convert a small-amplitude AC signal into a square wave of 11.2VDC.
Seems like a straightforward problem. . . Unfortunately, I’m a Chemical Engineer, not an Electrical, so phrases like ‘power supply current drain is independent of the magnitude of the power supply voltage’ or ‘input common-mode’ defy my understanding. Doing some reading, a ‘comparator’ seems like it would do the trick – when the sine wave is ‘positive’ relative to ground send the full saturation voltage to the output. Then stop when the sine wave goes to 0 or negative. Radio Shack has the LM339 quad comparator in-stock locally – using this chip, I devised the circuit in the attached sketch. Apparently there also needs to be a ‘pull up’ resistor? Not sure how I would size that, so I’ll ask for help!
Thank-you and please comment. I’d rather learn something about electronics than put a hole in the bottom of my boat for a new speed sensor.
Thanks for the reply - the speed sensor's output is speed-dependent. If I read the specs correctly, for the range of interest, the output voltage will be between 0-8 volts peak-to-peak (and will vary with frequency from 0-48Hz).
That is, the frequency is 6Hz/knot and the voltage output is 1V peak-to-peak/knot -- and I would be 'screaming' at 8 knots!
Thanks for the reply - the speed sensor's output is speed-dependent. If I read the specs correctly, for the range of interest, the output voltage will be between 0-8 volts peak-to-peak (and will vary with frequency from 0-48Hz).
That is, the frequency is 6Hz/knot and the voltage output is 1V peak-to-peak/knot -- and I would be 'screaming' at 8 knots!
Thanks, Eric. Yes, the boat is a 1975 Sabre 28 -- length at waterline is just under 23 feet, so theoretical hull speed is, of course, actually under 8 knots.
The schematic makes sense -- two questions:
* What does the notation: "wiper = 0.25" mean? Some 'base' resistance in the variable resistor?
* Assuming I get the pinout staight, should I be able to use the quad LM339 in place of the LM393?
Thanks, Eric. Yes, the boat is a 1975 Sabre 28 -- length at waterline is just under 23 feet, so theoretical hull speed is, of course, actually under 8 knots.
The schematic makes sense -- two questions:
* What does the notation: "wiper = 0.25" mean? Some 'base' resistance in the variable resistor?
* Assuming I get the pinout staight, should I be able to use the quad LM339 in place of the LM393?
Thanks! FYI, this project is to connect the sender paddlewheel from a 'Signet Marine' SMARTpak SL80, to a Raymarine ST40 Bidata display.
OT: If I'm not mistaken, I believe the 25 foot westerly is similar to the Sabre 28. Kitchen is in a different spot, but both are nice cruising boats that feel bigger than they are. Good headroom down below and nice layouts.
I have a slightly different situation with an old speed trasducer that has a magnet on the paddlewheel that closes a reed switch contact each revolution. so 0V-12V pulses are generated in a similar way to a hall effect paddlewheel, BUT I think the pulses must be noisy. Modern pulse counting type speed instruments connected to this paddlewheel give erratic readings. I tried a simple RC filter to maybe subdue some of the noise, it seems a little better for certain speeds, but in the end is still unusable.
I hate to replace the transducer with a modern one as the current one is narrow, and has a special mount on the centerline so that it reads the same on both tacks. Can't do that with larger diameter new transducers.
Do you think the comaparator circuit might clean up the edges/spikes of a mechanical reed switch transducer so that olny the main pulse is seen?
And, will the comparator turn off if the signal drops to zero but does not go negative?