View Full Version : GPS Speedometer / Digital Tachometer

05-03-2006, 05:01 AM
I just finished rebuilding my boat last weekend and the only two things that I haven’t installed in it are the speedometer and the tachometer. I was wondering about a previous post here at parallax·about a BS2 controlling a car's ignition system. Some people kinda·said yes the BS2 is fast enough and others said no it’s to slow. But instead of precision pulsing 6 – 8 I/Os, i would read only one thats linked to pulse with·the ignition of a singal·cylinder. the·BS2 calculates the RPMs of the engine with that singal pulse. let me also say that the engine im using is an in-line six cylinder two stroke (Bad azz motor),·but its·twice the combustions·for every two·revolutions when compared to a car. But Im almost sure·the BS2 could easily handle those kinds of speed,
my other question is about·accurately tracking the speed of a GPS antenna. I see it all the time in the moderately priced·GPS units, and their usually dead on (some go down to like 0.01 Mph. but those things·measure almost centimeters.) The 0.1 - 1·range will have to do for me though. you all think it would be·too huge a·project to have a BS2 read coordinates and calculate them to equal speed? im·not really sure how i could interface·to a·GPS. i dont even know how fast all of it would run. i would need it to be fast enough to look almost fluid; not·skip large amounts of digits under hard acceleration. but im sure there will be some processor overhead and calculation delay.

Post Edited (Grant_O) : 5/3/2006 3:37:11 AM GMT

05-03-2006, 07:09 PM
Look in to Parallax's SX chip.
Fast stuff....and cheaper!!

Use one SX for the signal pulse counting and another for the GPS....then have a 3rd getting the data from both and dumping to an LCD (could probably use a normal stamp on that one).

As far as the speed of the pulses, you certainly don't need to count pulses on every single cylinder (unless you want to detect missed sparks)....you could cound 1 cylinder and do the calculation like you said.
What's the max RPM you'd be after? MAX RPM/6cylinders will give you the number of pulses on a given cylinder (is the math right?).

How are you going to "detect" the pulses? Some guys have used inductors to pick up the bangs from spark plugs....I think I heard someone using a hall effect sensor (not sure how that worked out). In any event, you'll probably want to look at opto-isolating your uP's. An opto-isolator is pennies, where an SX is a few bucks and a stamp is more.



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05-03-2006, 07:12 PM
Getting speed from the GPS is the simple part. Your stamp will just use the SERIN command to read the string from the GPS and pull out the speed number. Most GPS's units put out several different strings, so you have to check the documentation to see if the unit in question has what you need. The update time can vary depending onthe GPS. Usually its no problem to update at least once per second.

Measuring engine speed could have a few issues. If your engine is fairly new there may already be a tach-out signal in the ignition module. It is usually used as part of the rev-limiter to protect the engine if the prop comes out of the water. You could also connect an opto-isolator to one set of points (if you have them) and generate a pulse stream for the Stamp to measure. Or, get a Hall effect sensor and monitor the primary current pulses in one coil circuit.

When all's said and done, you may be forced to convert the pulses into a real analog voltage and measure it with an A/D tied to the Stamp.