I need electronics assistance By glavey (Read 533 times)

[glavey]

I am planning on trying to make an analog tachometer powered by an arduino with a stepper motor moving the needle for my scooter. I need a way to sense the rpm of the engine and turn that into 5v pulses that the arduino can read. A few ideas have come to mind.

Idea#1 is using a reed switch but I dismissed that almost immediately because I doubt it could close and open fast enough at 8krpm.

Idea #2 is wrapping a wire around either the high tension spark plug wire or the low tension coil wire.

Idea #3 is using a hall effect sensor to detect the magnet on the exterior of the flywheel. I have a feeling that the magnets inside the flywheel used to energize the stator will cause the hall effect sensor to give a noisy signal.

Idea #4 is tapping into the ignition trigger wire that goes to the CDI. I have been informed in a thread I created on another forum that I should use a oscilloscope to measure the voltage coming from it to make sure I don't fry my arduino. I don't have an oscilloscope, nor do I have the funds to buy one. All I have is a few multimeters and I know those won't update fast enough to catch the voltage spike.

Anybody else have any ideas on how to get an rpm signal from the engine?

[cyborg]

i'd use the wire wrap method,,,,BUT you have to be careful with the wire gauge and number of wraps,,,i'd research boule's theory for power production to determine this,,,there's probably someone out there that has a kit or instructions on how to fab up the "sensor" you're after i would think

[Bashan]

Odd you should bring this up. Yesterday I was working on my wife's bike (IT'S A 150!!) and I had the top cowling off. The tach had quit working about 6 months ago so I thought I'd see if I could fix it while I was working on some other things. I had wondered where it got it's signal and discovered a Y/B wire into the back of the tach. I metered it and continuity was to the positive feed wire on the ignition coil. Apparently the tach is (or was) capable of interpreting the signal. You can get a voltage signal off of this wire, I'm not sure about pulses. However, it fires every rev even though it's a 4T. How about the trigger coil? Low voltage discrete AC current fires every rev. It's not noisy as far as I know, it is triggering the CDI, it can't be too dirty. How you would translate that into RPMs I have no idea. But I know many tachs in fact work off of the trigger pulse.

[Alleyoop]

[glavey]

Jul 18, 2014 11:48:48 GMT -6 Bashan said:

...I had wondered where it got it's signal and discovered a Y/B wire into the back of the tach. I metered it and continuity was to the positive feed wire on the ignition coil. Apparently the tach is (or was) capable of interpreting the signal. ...

That is the same way my cheapo chinese analog tach gets its signal. Is that wire ac or dc and what kind of voltage is it?

[cyborg]

Rich i would think radio shack has a small cap that would clean up the signal,,,,no?

[Bashan]

Cap would probably do it...yes. Here's the coil feed from the CDI being tested in series. It's AC voltage, one probe in the coil feed, ground the other. The value should be 5 to 30% of the AC feed to the CDI. Key is on and kill off so spark is not grounded but the bike won't start because you're bypassing the ignition coil, crank the engine. So if the stator is cranking 100v AC on the red /black your coil feed should come in around here:

That is mega amplified on the secondary coil in the ignition coil to about 30K to 50K. But that would only apply to the tachs like a tiny tach that get the induced current in the wrap around the plug wire. The coil feed is of course pre ignition coil current.

[glavey]

If you remember, my stator had 2 coils for the CDI; would that mean more voltage and/or current for the CDI and thus for the coil?

I'll test the output tomorrow.

[Bashan]

The voltage would probably be the same, I've never heard of much over 100v AC for the red/black. You've got to remember these CDIs are made for a certain input from a wide variety of bikes. They never ask what your stator AC CDI voltage is to send you a specific CDI. Possibly, it would have more milliamps, more current, more electrons moved through one spot during the same time period. It would depend on how they're wired together too. That may or may not increase the coil output, it runs through several filters before going on to the ignition coil. Again, these things are designed to work around certain parameters. You're making my brain hurt, these are things I rarely think about. Do I sound like Tesla!?? Good grief, I'm going to the basement to continue my experiments....um...er...projects. Do the testing and report back!! I shall otherwise be occupied in the laboratory.

[glavey]

That guy's smile made me mustache. I mean... damn my dyslexia.

I'll get to testing right after I eat some year old taco shells with salsa.

[glavey]

The voltage on the yellow/black wire is 40v AC, red/black wire is 85v AC.

[Alleyoop]

All is good there drive on

[glavey]

I'm starting to think I'll just use a hall effect sensor with the external magnet on the flywheel. I won't need any conditioning circuitry, they are quite temperature tolerant, and I can get them real cheap on ebay.

[Bashan]

Well, there goes my consistent voltage theory. Apparently your big time testosterone driven two magneto coil feed to the CDI produces more voltage than Alabama Power. That's fine, send us some juice via the grid. That kind of voltage on the Y/B makes sense considering the input. I'd like to see pix of components and a schematic when you get done.

[glavey]

I absolutely will do that. I'm all about sharing knowledge. I just ordered a large batch of stuff from ebay, including (hopefully) everything I will need for the tach. Here's a short list of the items that will go into the tach:

Arduino #1
Arduino #2
Hall effect sensor
Stepper motor driver board
Voltage regulator/DC-to-DC buck converter

I'm getting all tingly thinking about this project.