Spark plug wire inductive pickup for RPM 4

[ziploc]

This is kind of a repeat of a very old thread:

http://www.eng-tips.com/viewthread.cfm?qid=223731

I'm looking to read RPM on my paramotor (120cc two-stroke engine) with an Arduino. The part I'm particularly curious about is sensing the spark and conditioning that signal. As mentioned in the older thread, there are very simple tachs on the market that simply use a wire wrapped about 4 times around the spark-plug wire (with one end completely free). I've seen a few approaches to conditioning the signal, but it sounds like "itsmoked" had a slick approach that unfortunately didn't appear in the old thread. Hopefully he can chime in here.

Thanks in advance for any tips.

 

[ziploc]

>> If it doesn't work, come back. I'll figure out something.

Thanks! I really appreciate it. Unfortunately, it might be a few days - or weeks - before I get to that part. I'm juggling a few projects, and just trying to make sure I have the circuits and software worked out for this part. I've got the Arduino talking to a 20x4 LCD display, and tonight I plan to hook up the barometer chip to give me absolute altitude, relative altitude, and vertical speed.

>> What else does this do besides monitor the tach.

The plan is to do altitude, vert. speed, RPM, CHT, EGT, fuel level, and engine hours to start with. I just like the idea of making a single "dashboard" rather than stringing a bunch of instruments along my throttle cable.

I've also got to make that downwind cart go directly upwind faster than the wind in the coming weeks.

Thanks again.

 

[Skogsgurra]

Learned about modern lawn-mowers today. Maybe I should get one without points.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ziploc]

>> Learned about modern lawn-mowers today. Maybe I should get one without points.

I wouldn't risk it - not if you're going to be mowing the same old lawn

 

[ziploc]

I've got the LCD display and barometer hooked up and putting out altitude. Our NASCAR ignition system is coming back from the field, so the RPM sensor is next. Thanks again for all the good info.

 

[ziploc]

I got a chance to wire up the simple circuit and test it today. I operated the ignition system at various speeds up to approximately 16,000 RPM.

The circuit appears to work well, but has significant ringing. Interestingly, it seems it gets the initial pulse, then a brief quiet period, and then the ringing. The ringing does appear to have a fixed duration and offset from the initial pulse. It's not dependent on RPM.

Here are images of the circuit and the scope trace.

http://tinyurl.com/7hnfej7

http://tinyurl.com/855hw7s

I'd rather not solve the ringing problem in software as I expect it will waste a lot of cycles. I would think I could "debounce" it with a 555. But I'd love to get some suggestions from the experts.

Thanks again for all the help.

 

[OperaHouse]

That looks unusual like an old MSD-5 unit. The gain may be too high so that traailing ringing is still picked up. What did you use as a bias resistor? To lower sensitivity try a 100K pot connected to common with a .1uF cap in series connected to the base. Adjust it to see if the waveform changes. That may be enough. As long as the transistor goes into near ssaturation, a small cap maybe 100pF from collecor to common will stretch out the pulse as well as a 555. Oh, and don't mention a 555 in my presence ever again.

 

[ziploc]

MSD-5 unit? I had no idea what that even was. We just built a small simulator consisting of a distributor and coil, powered by a 12V battery and turned by a cordless drill. The distributor goes to an 8-cylinder engine, but we only use the points in it - not the rotor. So the coil fires the same spark plug every time the points open.

>> The gain may be too high so that traailing ringing is still picked up.

Could that mean I have too long a sensor wire along the spark plug lead?

>> What did you use as a bias resistor?

The resistor marked 2-5K on the diagram is a 4.7K. The one marked 1M or 2M is 1 Meg-ohm.

>>
To lower sensitivity try a 100K pot connected to common with a .1uF cap in series connected to the base. Adjust it to see if the waveform changes. That may be enough.
<<

Thanks. I'll give that a try.

>>
As long as the transistor goes into near ssaturation, a small cap maybe 100pF from collecor to common will stretch out the pulse as well as a 555. Oh, and don't mention a 555 in my presence ever again.
<<

Cool. I'll try that too. Did a 5XX once touch you in a way that made you uncomfortable?

 

[OperaHouse]

MSD is a company started at least in the early 70's and still exists. I bought a MSD-5 (current product MSD-6A) for my Mazda rotaty that would eat points by about 4K miles. It now is running the flat head in my 1955 Correct Craft. It stands for Multiple Spark Discharge and fires the plug 5 times in a row. I can run the boat at 300 rpm for hours at festivals without fouling a plug.

The amount of ringing should be fairly constant. Reducing the gain should get you to one or two rings and small collector cap would stretch those two pulses together.

 

[ziploc]

Very interesting. I used to have a 1974 Mazda RX4 when I was a wee lad. It had the 13B engine and a distributor with two sets of points. Each rotor had two spark plugs that fired one right after the other in rapid succession - but they didn't do multiple sparks for a single "stroke". Never had any problems with eating points.

>> Reducing the gain should get you to one or two rings and small collector cap would stretch those two pulses together.

Thanks for the tips. I'm hoping I can try that today. Unfortunately I'll be traveling Friday and all next week - so there will be another break in my progress. Fortunately this is more hobby and learning. There's no deadline.

Next up on the list is sensing fuel level. I'm thinking capacitive right now, but I've got a ridiculous number of different ideas including optical (index of refraction) and pressure.

 

[OperaHouse]

Mine was a 74 RX2, books say they never made one that year. It was tuned to factory specs but either from a change in emissions standards or it was just made wrong. The rotor was a couple degrees off from the point cam. Not only did it eat points but plugs, 10K and it needed new ones. I eventually soldered a short length of wire to the rotor and that solved everything. Car is long gone but still have the engine 50K. Yup, they gave me a new one after 5 years.

Does the vertical decent screen say GOODBYE after a certain point?

 

[ziploc]

Interesting. My RX4 specified NGK B7EM plugs. It worked great with those plugs. A couple of times I used the Champion equivalents. Ran fine with them too - but used them up in no time.

>> Does the vertical decent screen say GOODBYE after a certain point?

I'm not sure what you're asking?

 

[OperaHouse]

If it is the LAST screen the pilot will ever see, you ought to do something special.

 

[ziploc]

>> If it is the LAST screen the pilot will ever see, you ought to do something special.

Ah yes of course! It says "shut up and die like a man!"

This comes from a true story incidentally. We usually communicate by ham radio when hang gliding in the mountains. A few years back there was a pilot that had got blown back. He was now low and looked like he was going to be landing in some very unfriendly terrain. He was on radio constantly narrating his worsening situation. When he finally took a breath, one of the other pilots got on radio and said "shut up and die like a man!" It can be a rough crowd sometimes.

 

[LiteYear]

We just built a small simulator consisting of a distributor and coil, powered by a 12V battery and turned by a cordless drill. The distributor goes to an 8-cylinder engine, but we only use the points in it - not the rotor. So the coil fires the same spark plug every time the points open.

Maybe this is the cause of the second ringing? Since the high tension circuit is left connected, could there be a second firing when the point re-closes? Seems plausible to me from a circuit analysis point of view but depends on the ignition design.

I suspect the larger ringing is actually the spark you're after and the smaller ringing is the secondary spark. As already discussed, this suggests the circuit is currently too sensitive. Either reduce the sensitivity (lower the input resistive divider ratio or reduce the number of turns on the sense wire) or slow the response (add capacitance).

 

[ziploc]

>> Maybe this is the cause of the second ringing? Since the high tension circuit is left connected, could there be a second firing when the point re-closes?

I don't see how the points closing could cause a second spark, but I could be wrong. Also, the time between the first and second set of pulses does not change with RPM.

I didn't get a chance to try the suggestions OperaHouse made today, but hopefully tomorrow. Otherwise it'll have to wait a week.

 

[ziploc]

Yo OperaHouse - on the plane back east last night I decided to see if I could understand the circuit you gave me. I thought it made sense, except the diode was reversed relative to what I would expect. I would have thought you were using the forward bias voltage to establish a voltage level on the base of the transistor. Can you walk me through it?

Thanks.

 

[OperaHouse]

That diode protects against revers voltage. The transistor operates in a starved condition which makes it more sensitive. That is the reason the collector is normally at 5V. Note the resistor is dotted in as an option, you may want to eliminate it. At the times I write posts things may be glossed over. I just wrote your longer post and it was eliminated when my finger glanced over the pad.

 

[ziploc]

Thanks very much for the explanation. It occurred to me that the diode would protect against reverse voltage, but I'm not familiar with the downside of bringing the base to a negative voltage. I thought that was no different than bringing it to ground.

I presume there's no downside to bringing the base to to high a positive voltage?

 

[ziploc]

I think I may have found an answer to my questions by poking around on Google. It looks like the BE junction will zener at something like -7V, and no need to limit positive voltage as long as collector current is limited.

This is very interesting stuff, that I've always wanted to learn. Maybe this is the time

 

[ziploc]

Alrighty - a new (and now obvious) factoid was pointed out to me... No need to limit the positive voltage to the base with a diode - since the B-E junction IS a diode.