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  #21  
Old 02-01-2011, 04:16 PM
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Originally Posted by kibble View Post
Good news! I was just playing around with the input part of the circuit and it works! Obviously you can't see it working in the pic, but the yellow wire is wrapped around the plug wire and the voltage induced when it fires is high enough to be amplified by the OP amp. I have an LED on the breadboard that lights up when the coil fires. I tested it out just a few minutes ago and it works great!

I need to test it out with an engine I can bring to my workbench so I can use my scope on it to make sure the pulses coming from the chip are wide enough and if not, then adjust resistor values accordingly.
Sweet! Nice work
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Old 02-01-2011, 04:19 PM
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Sweeeet! So right now, it's just the input side, right? read only?
Awesome- I figured a few days before you got the ball rolling.
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  #23  
Old 02-01-2011, 04:27 PM
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Yeah, the output side doesn't require much detail in terms of components. It's the input that I was trying to figure out how to do since trying to bring down the high voltage from the plug is risky and could fry a few things. This method is much safer, doesn't require having to wire anything directly to the plug and it's easy to just wrap a wire around the big cable when installing.
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Old 02-02-2011, 08:38 AM
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This is super cool, I'm following this thread with great interest Just a suggestion, why not scavenge some parts (especially the spark pickup) from one of these -->
The pickups tend to be fragile though, I've cracked the ferrite pieces inside just by dropping the pickup on the floor, so perhaps not so great for off-road applications. Must the pickup wire be wrapped around the plug lead, or would a big ring terminal work?
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Old 02-02-2011, 01:43 PM
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I thought about using some sort of ferrite pickup that could be zip tied or something, but like you said, it tends to be fragile and chips easily. Wrapping the wire works quite well and I actually discovered that my circuit is sensitive enough to pick up the AC current going through a regular 120v power cable! A ring terminal might work, but the purpose of wrapping the wire around is so that it forms a coil that will pick up on the magnetic field around the plug lead and produce a small current that is detected by the circuit. I'm working on wiring the microcontroller at the moment. Then I'm gonna figure out how to program it so that it can calculate RPM from the input pulses.
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Old 02-04-2011, 07:42 AM
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Quote:
Originally Posted by kibble View Post
A ring terminal might work, but the purpose of wrapping the wire around is so that it forms a coil that will pick up on the magnetic field around the plug lead and produce a small current that is detected by the circuit.
I kinda figured that's what you were going for, but wasn't sure. I remember seeing a system on an old biplane when I went to turbine school, to drive the tach it measured spark voltage thru the plug as it returned to ground, using a big ring terminal/washer between the plug & cylinder head. Not sure why they did this, and I'm not sure how well it worked, but it popped into my head immediately when I read this thread. Glad to hear your system works though! Keep up the good work!
  #27  
Old 02-05-2011, 07:14 AM
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Hey, Kibble- I just had a thought (which happens periodically)

Since my limited knowledge is, well, limited- I'm wondering if a bit of a redesign might be in order. Here's what I've got:

We (I?) want the circuit to sense an over-rev and kill the coil until the revs drop, and then allow ignition again, right? So if the pickup is sensing from the spark plug lead, how will it sense that the RPM has dropped to a safe level?

In my mind, once the coil is grounded out, there is no signal for the circuit to read, thus keeping the count going.

Is there a way to detect the pulses from the "low" side of the ignition coil?
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  #28  
Old 02-05-2011, 08:21 AM
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Yeah, good point. But I think it would still work. It all depends on how the circuit determines an over-rev condition. If it takes only two spark plug pulses to determine whether the engine is out of range, then I think using the spark plug wire to get your reading is a great idea.

If two pulses are too close together, causing the device to cancel the next one, the next pulse will be within the tolerable limit, because the period will be twice as long. This will cause the device to allow the next pulse. These two would represent a standard, uninterrupted reading again. If RPM is still too fast, it will again read too short of a time between that pulse and the next one, which will cause it to cut out again for one pulse.

The effect would be that if you were near the red-line RPM and started coasting downhill, the limiting device would eliminate every third power stroke.

I suppose you could program it to turn off for half-second intervals instead, or whatever you want. But sensing the time between every two pulses seems like a very good idea to me. When the limiter is activated and makes the plug fire on two out of three power strokes, the extra fuel wouldn't accumulate. It would be burned off right away, which wouldn't happen as well if your device were programmed to cut off the spark signal for some longer period.
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Old 02-05-2011, 09:49 AM
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I think this is a really cool project. As far as I know, even the super expensive race kart tachs dont have the rev-limiter feature.
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Old 02-07-2011, 12:48 PM
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I was thinking of just grounding down the coil for a fraction of a second kinda like robbie said, releasing ground, taking another reading, calculating RPM and either allow to continue if below or at limit or cut off if above. I'm guessing the result should be similar to what happens when you hit the limiter in a car and the needle bounces off redline. All this would happen in a really short period of time and which is why a microcontroller is ideal for this soft of thing. The biggest benefit to using a microcontroller is that because the control functions are all in software, the hardware need not change much, if at all. If something isn't working right, I'll reprogram it to do what I want it to and change timings around.

Using a sensor that takes a reading off the flywheel would be more accurate, but that would require installation to be a bit more complex. I'm trying to keep it as simple as possible while still providing desired results. With a setup like this, a max of 3 wires would be required; GND, coil lead and coil shutdown and the unit can be powered by a 9V battery.
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Old 02-07-2011, 03:44 PM
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Can't get much more simple then that. I'm loving this idea.
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Old 02-09-2011, 08:46 AM
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I'm using an AVR chip to do something like this. The input side is pretty much the one described in the following link, based on an LM324 to do signal prep: http://www.avrfreaks.net/index.php?n...topic&p=410615 (you have to log in to view the schematic). Note that if you are doing true frequency counting, you have to disregard the ringing in the signal at each fire, also as described in the post.

The shutoff is a avr driven servo linkage to the throttle / off switch.

I'm about 300 miles from this deployment but I'll try to grab a photo soon.
  #33  
Old 11-11-2011, 08:13 PM
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this ever go anywhere, fellas?
  #34  
Old 11-14-2011, 11:10 AM
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I know this is kind of dead, but maybe we can revive it some...

My idea was to pull the signal off the coil wire and use a adjustable pot to use it as a dial to be able to set the rpm max, then basically have the system ground the spark plug once the pot had enough ohms from the inductive wire on the coil, thus like making a killswitch where the coil grounds to the block. You can make pot fire a led if you wanted and use a reed switch as the ground wire trigger, so it would be open normal under your set RPM, then once the pot gets enough signal to fire through the LED, that would trigger the reed switch to close thus grounding the spark. You would have to run a wire inside the plug cap I guess or kill the spark or just not run a plug cap and have a spark plug with the top that will unscrew off with a wire that can be grounded...
  #35  
Old 11-14-2011, 04:28 PM
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Quote:
Originally Posted by Justin9 View Post
I know this is kind of dead, but maybe we can revive it some...

My idea was to pull the signal off the coil wire and use a adjustable pot to use it as a dial to be able to set the rpm max, then basically have the system ground the spark plug once the pot had enough ohms from the inductive wire on the coil, thus like making a killswitch where the coil grounds to the block. You can make pot fire a led if you wanted and use a reed switch as the ground wire trigger, so it would be open normal under your set RPM, then once the pot gets enough signal to fire through the LED, that would trigger the reed switch to close thus grounding the spark. You would have to run a wire inside the plug cap I guess or kill the spark or just not run a plug cap and have a spark plug with the top that will unscrew off with a wire that can be grounded...
As "he who issued the challenge", I still would love to see someone put together a working prototype. I guess Kibble has been otherwise occupied- life has a way of doing that to you.

As far as the funkadelic plug boot/cap, could you not re-arrange the circuitry to ground out the primary side of the coil?
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  #36  
Old 11-14-2011, 05:14 PM
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have not thought about that but you very much could... so that is another angle... I can't work on this right this moment, but would like to try it soon... but I wanna to throw the idea out there and see if someone else could tackle it... most fo the pieces can be purchased at a local radio shack...
  #37  
Old 11-14-2011, 05:24 PM
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My limited electronics knowledge (if sufficient to begin with) is now rusty as he//. Otherwise, I'd attempt this myself. I just don't have time for the amount of research I'd have to do to bring myself back up to speed. Too bad, too- because I still have a variable power supply and breadboard, and a decent fistful of components including some assorted chips and pots...
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