Best way to read RPM on an engine.

mosquito56

· What is the best way to read the rpm on an internal combustion engine with mags?

· 1. coil around sparkplug wire?
· 2. connection to magneto.·

· I have no ideas on the outputs of either of the above ideas and how to setup a voltage that won't fry the prop.


· Any ideas would be appreciated. Thanx Don

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······· "What do you mean, it doesn't have any tubes?"
······· "No such thing as a dumb question" unless it's on the internet
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virtuPIC

Many years ago Elektor electronics magazine had an issue with many car electronics projects. They used your solution 1, a few windings around the spark plug wire. But take care to handle possible overvoltage!

For option 2 you need a magneto that delivers voltage depending on RPM. In a plane you have such a beastie. In a car you only have a generator delivering voltage at a frequency that is proportional to the engine's RPM. So you won't get the phase if you want this, too. And you also have to take care of overvoltage protection.

There are other options. Paint a few stripes on the shaft and use an optical sensor. Use a magentic sensor near to a tooth wheel.

virtuPIC

P.S.: Haven't I seen another thread on this topic recently? Search this forum.

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Beau Schwabe

mosquito56,

For option #2, look on page 9 of the Propeller QuadRover Robot documentation to see how it was done there.

www.parallax.com/Portals/0/Images/Prod/2/281/28195-QuadRover-v1.4.pdf

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Cole Logan

What type of engine are you using. Some distributors have a tach output. The other thing you can do is hook an opto isolator to that negitive side of the coil.

mosquito56

· Beau thanx for the info it's right on the mark. Just a couple questions.

·· 1. The input pin2 grnd? in connected to pin 1. Is this a shielded cable. I am not familiar with this hook up.
·· 2. The bat54s is a cuircuit to elimate excessive voltage at tach output?
·· 3. What does the output at tach look like. Since it is connect directly to the prop it looks like it might be 3.3v pulses of some sort.
·· I include a schematic in express sch format.


BTW: I am using a 6cyl aircraft engine made in Australia call the Jabiru3300. I cannot find any info on tach output.· I am going to start researching opto isolators thanx for the idea.

Thanx to all the input. Don

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······· "What do you mean, it doesn't have any tubes?"
······· "No such thing as a dumb question" unless it's on the internet

Beau Schwabe

mosquito56,



I'm not sure I understand your question in #1, the magneto will have two terminals one (usually the outer case) is connected to GND, the other would connect to the 470K resistor in the schematic.

The BAT54S is a pair of Schottky configured in a reverse bias mode, this serves to act as a clamp for any voltages above 3.3V or below ground.

As far as what the signal looks like, yes it is a 3.3V pulse, but the duration of the pulse width will vary depending on how your engine is configured.

With the attached image, I simulated a 50kV magneto pulse every·.06 seconds (~1000rpm) with a duty cycle of 12.5%


Reference:
www.konkoly.hu/staff/racz/SziderikusOra_elemei/BAT54S.pdf

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 3/9/2009 6:22:13 AM GMT

mikediv

mosquito56, if you have a Vertex or MSD with the mechanical tach output summit I am sure others Jegs sells a very cheap mech to electronic output device under 40 bucks that works with Vertex, MSD there are two wires red and green
green is the ground and red is for a tach output 12V dc sqaure wave if you have dual tachs just use the drive tach out , there is a bunch of designs google or yahoo tach circuit I wired up a basic stamp to store my RPM totals on runs and graph it to excel oops didnt read whole thread , there is also a product called tiny tach works with wire wrapped around number 1 for 4,6,8 cylinders you can do the same thing with its output if you want to store info

mosquito56

···· Thanx mike for the input but I'm doing an engine display to a t.v. in the plane. Very limited dash space so another instrument is not an option.



· Beau, thanx for the help. I will be using your idea.



· Don

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······· "What do you mean, it doesn't have any tubes?"
······· "No such thing as a dumb question" unless it's on the internet
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Drone

What is this "Integrated AC Generator" that's included in this Jabiru 3300 engine? Is it an alternator? If-so, there you go... A simple way to sample and convert back to RPM. But I may be missing something obvious; I Only spent a few minutes drilling on the Web about this engine.

Awhile back I stumbled across a little project that used a PIC or AVR to project for a little device to display RPM in a car. You plug it into the cigarette lighter and it filters and samples the alternator "whine" in the audio range then converting to RPM display on a little LCD. Can't find the link though... Sorry.

Rgds, David

Ken Gracey

mosquito56,

Here's a circuit I used on a mini Honda engine with magneto:

http://forums.parallax.com/showthread.php?p=612409

Ken Gracey
Parallax, Inc.