roll your own tach

[stderr]

I've been tinkering with a microcontroller demo board, trying to see how hard it is to make a tach. I'm finding the tough part is the sensor for picking up the spark. I tried a Hall effect sensor, but that was just a little to the right of useless. It worked, sort of, if I held it right up against the flywheel cover. I had hoped it would pick up the magnetic field around the wire during the spark discharge, but the only thing I could get a reading from was the magnet on the flywheel.

I recall reading a thread here on AS about a similar development, but now I can't find it .

Here's a video of a little Shindaiwa 360 idling, with a scope picking up the pulses from the plug lead, and a screen-shot of the displayed data at the end (the display is really hard to read unless you view it in a higher resolution):

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So far, I have it working from 300-30,000rpm (the software, not the saw ). Changing the range is a matter of software at this point. I also have 2/4-stroke switching, and multiple cylinder selection.

I'd love to hear some opinions about whether it's worth working into something useful, or features you wish for. I know a tach can be had for peanuts, but I'm still curious about how it all works. If anyone has tips on how to properly pick up the spark pulses, I'm all ears.

 

[Arrowhead]

Your computer/electronics skills are impressive. I know how to use my tach, but don't know what makes it work.

 

[beavis331]

Roncoinc had a thread a while back dealing with this. It was very interesting.

 

[stderr]

Roncoinc had a thread a while back dealing with this. It was very interesting.

That was the one, thanks!

 

[Dan_IN_MN]

Roncoinc had a thread a while back dealing with this. It was very interesting.

Here is the thread

 

[roncoinc]

Yup,that had promise but i cant write code and the developer went on to other things..
You need an amplifier is all.a small RF amp.filter it.try to keep it linear.dont try to amp to much.check with the scope to see what your getting for voltage from the pickup now and maybe double it with the amp to start with.
try an antenna wrapped around plug wire.
try diff sampling rates also,may find a sweet spot..in the end 2x a second would be a nice rate.
good luck

 

[WVshooter]

Bump.

Any groundbreaking developments here?

 

[srcarr52]

Hell effect sensors are for close proximity use only (within 0.5") and they are more designed to read the square wave induced by a passing magnet.

Like the other thread you're going to want to think of it more as picking up the radio wave from the emp of the coil wire. Use an antenna and take the time between say the top 10% spikes (I think the other thread had a potentiometer to tune the cutoff threshold).

A couple of cool additional features would be a stop watch and data logging. It would then be the ultimate tach for cookie cutters.

 

[stderr]

Slow Progress

Hell effect sensors are for close proximity use only (within 0.5") and they are more designed to read the square wave induced by a passing magnet.

That's what I understood, but I thought it was worth a little experiment. I thought there might be enough magnet field around the plug wire during the spark discharge to pick it up. I learned that this is not true.

Like the other thread you're going to want to think of it more as picking up the radio wave from the emp of the coil wire. Use an antenna and take the time between say the top 10% spikes (I think the other thread had a potentiometer to tune the cutoff threshold).

This is the most interesting aspect of the whole thing to me. I can't wait to get back to experimenting on this part.

A couple of cool additional features would be a stop watch and data logging. It would then be the ultimate tach for cookie cutters.

Cool feature, I like that. How would you start and stop with your hands on the trigger and handle? Foot pedal? Push button mounted to handlebar? Automagically based on RPMs?



Where I'm actually at:

Christmas got squarely in the way. I also had a neighbour come by and unload on me about the noise. My first thought was to thumb my nose at him as I was running it at 1:30 in the afternoon on a Friday. Neither one of us is going anywhere, so there's little point in starting a war over it.

I need a more quiet way of testing, not to mention the garage is unheated and it's getting old fast working out there at this time of year. I don't mind the cold, so long as I'm moving around a bit. To keep it alive, I've been putting together a little jig for spinning the flywheel up to full speed indoors, and plan on mounting an ignition coil to that. Once I get that done, I can get back onto the tach itself.

 

[Dan_IN_MN]

Keep us informed! I'm VERY interested! I want a tach, however, there just isn't the extra funds right now to buy one "off of the shelf".

If you're looking for someone to "test drive" a kit or to check out the instructions I'm willing.

 

[srcarr52]

Cool feature, I like that. How would you start and stop with your hands on the trigger and handle? Foot pedal? Push button mounted to handlebar? Automagically based on RPMs?

For the prototype it could just be two buttons on the top corners for start/stop and reset. Just like a stop watch. Later it would be nice to have a remote micro switch that you could mount on the rear handle and hit with your thumb.

You'll also need a button to start/stop data logging if you go that route.

I need a more quiet way of testing, not to mention the garage is unheated and it's getting old fast working out there at this time of year. I don't mind the cold, so long as I'm moving around a bit. To keep it alive, I've been putting together a little jig for spinning the flywheel up to full speed indoors, and plan on mounting an ignition coil to that. Once I get that done, I can get back onto the tach itself.

You can just spin an old saw with a drill with the spark plug out (but ground it for the resistance). It will get you up in the 2000 RPM range for testing. If you have an old junky saw cut the rod off the crank and you'll be able to spin it faster.