Capacitive rpm meter - Page 1
Forum section: electronics / projects

author: senso
author_rank: Frequent Contributor
timestamp: May 09, 2013, 01:44:15 am
content: Good night, here I am seeking for your advice againI want to make a small rpm meter, the meter is based around a tiny2313 and some 7 seg dispays, I can hook it up directly to the pick-up and it works a treat but I want it to be a sort of hand-held device, and for that I would like to get the rpm signal from the high-tension cable that goes from the ignition coil to the spark-plug, there are meters on the market that measure the rpm via that way, and I would like to know if someone has a small schematic to do the signal conditioning to feed it to the micro, some years ago I saw one that was just a piece of cooper wire, coiled 2-3 turns in the cable, and I think it only had a pair of diodes to clamp the signal and a small transistor to buffer the signal, but I can't find it..Any ideas?

author: TheRevva
author_rank: Regular Contributor
timestamp: May 09, 2013, 03:39:41 am
content: Sounds to me like you already have all the basic concepts for the input signal conditioning.Just one small point I'd add is to make sure you 'shield' the case fairly well because under 'normal operating conditions', it will be in close proximity to a bunch of 'spark gap transmitters' <grins>.Taken to the worst case, one of my other 'hobbies' is drag racing. Consider a 'Top-Fuel' engine (those 8000+ horsepower engines that consume nitromethane as fuel)...These puppies use a pair of magnetos to set the fuel mixture alight... But here's the kicker... Each of these magnetos is generating about 50kV at somewhere near 44 AMPS!!! (Yep, that's like having a few dozen high-current arc welders). It's no surprise that, after each run down the track the spark plugs (there's 16 of them in a Top-Fuel V8, two per cylinder) are absolute toast! The side electrode is completely gone! (And that's only assuming nothing major went wrong!!!)As you can guess, there's quite a bit of RFI across the entire spectrum. (And that's why I'd HIGHLY recommend the inside of your case is shielded in some kind of Faraday Cage.

author: peter.mitchell
author_rank: Super Contributor
timestamp: May 09, 2013, 10:17:13 am
content: Just get an extruded aluminium project box and powdercoat it

author: SeanB
author_rank: Super Contributor
timestamp: May 09, 2013, 04:23:01 pm
content: Going to need a little more than 4 screws on the panels then, unless you use the one panel as a sensor.On the spark plug wear, my father did once buy himself an Alfa Sprint, and got the aftermarket ignition coil, Optronic ignition ( aftermarket coil ate points in a week) modules and found out that he needed special Lodge plugs to have any chance of having the plugs last between services. The regular single electrode plugs would do 1000km at best before they had lost the side electrode ( only made for hard starting, the coil was happy to fire the huge gap when warm) and were eroding into the ceramic. This did allow for a very lean mixture, and gave a very big power boost as well, such that it was a pocket rocket, capable of outdoing a regular 3 series of the same period ( when the Golf/Rabbit GTI MK1 was introduced as a low cost high performance vehicle) and still was economical on fuel otherwise. I do remember going up a long (20km) hill with him, with the car loaded with cement ( taking to a building site) and a 3 series BM coming past us in 3rd gear. He put his foot flat, and in about 20 seconds we passed the BMW, still in 5th and still accelerating from 80 to around 160 at the top of the hill ( no traffic cops on an uphill, always on the other side with the radar gun) with the BMW a small dot in the rear mirror. He liked that car, just it ate tyres ( funny that), plugs and was made, as far as we could tell, from compressed rust, despite the best aftermarket anti corrosion treatments and regular washing and waxing. plus the handbrake was only there as a afterthought, though with the massive inboard disc brakes it could stop in about it's own length in town. He sold the special tools he made for the weekly handbrake adjustment ritual with the car, otherwise what became a 5 minute job took an hour or more, as the adjusters were right down the front of the firewall next to the exhaust manifold, and needed a socket on the end of a half meter extension.

author: senso
author_rank: Frequent Contributor
timestamp: May 11, 2013, 03:08:29 am
content: Thanks for the suggestions, I think I don't need the metal box, as it will be placed far away from the coil, but thanks for the heads up.About the plugs, as all my engines are two strokes, eroded/melted plugs always means big expenses, because they can't run as hot, or as lean as a four stroke, and the spark energy is a lot lower, the coil is energized by a burst of 400v, and with just a measly tens os mili-amps at most, 44A is one hell of a coil, that thing must be the size of a teslaWhen I arrive to a working and reliable solution I will leave the project here in the forum,Nice to see some mother heads around here

author: G7PSK
author_rank: Super Contributor
timestamp: May 11, 2013, 08:34:31 am
content: Quote from: TheRevva on May 09, 2013, 03:39:41 amWow nearly 7000 horsepower just to drive the magneto's.

author: SeanB
author_rank: Super Contributor
timestamp: May 11, 2013, 09:49:32 am
content: Not DC, a short pulse every revolution, so probably only 2kW of power needed. Remember only 2 fire at any time, and there are only 4 firing pulses per revolution of the crankshaft. The 50kV is needed to fire the gap, but the arc discharge that it creates is only around 400V at peak combustion pressure, so the power dissipated is much lower. There probably will be as much power dissipated in the coil as in the combustion chamber from the spark, with probably 10% being done by the plug wiring and the distributor.

author: Rerouter
author_rank: Super Contributor
timestamp: May 11, 2013, 09:53:47 am
content: Here is the input i use for my counter, designed to handle the exact same thing, (and more) as your using it for only ignition coils you probably will not need the bias voltage, however in my instance it has to work off 700mV inductive signals up to pure coils and points,if you do also wish to use it for both, you will want to trim the bias to the point where the output just exceeds 2.5V, from there it will cleanly swing full scale to just about any signalEdit: woops 100p not 10n

author: Corporate666
author_rank: Unknown
timestamp: May 11, 2013, 07:48:36 pm
content: Quote from: senso on May 09, 2013, 01:44:15 amDo a google search on "MegaSquirt". It's a collaborative/open-source engine management system used by thousands of people - they have schematics available online and they have a tried-and-tested tachometer input circuit. It's very similar to one that I used on a commercial product I sold thousands of across dozens of different vehicles without any issues.

author: Zad
author_rank: Super Contributor
timestamp: May 11, 2013, 09:40:22 pm
content: You could alternatively take the pickup from the low tension side of the coil.

author: TheRevva
author_rank: Regular Contributor
timestamp: May 13, 2013, 01:35:24 am
content: Quote from: Zad on May 11, 2013, 09:40:22 pmThe OP mentioned it's a 2-stroke, and thus you'd probably find it's a magneto-style system. (The LT side of the coil is slightly 'uglier' than a traditional ignition system, but not by much)Having said that, if the engine is a single cylinder, or if it uses a single coil per cylinder with completely independent magneto inputs, then this is probably a god plan!

author: TheRevva
author_rank: Regular Contributor
timestamp: May 13, 2013, 01:47:40 am
content: Quote from: SeanB on May 11, 2013, 09:49:32 amPretty close to the money there Sean. The magnetos do NOT suck 7000 HP from the engine! (The engines _ONLY_ develop about 8000-10000 HP in total... Hang on, did I just say ONLY??? Gulp!)The only minor correction is that they don't fire 2 magnetos at exactly the same time. Yes, there ARE 2 plugs per cylinder, but the twin magnetos are phased to give a degree or two of timing between the 2 plugs on any given cylinder firing.Here's some other 'trivial facts' on these nitromethane gulping monsters...The supercharger sitting up top needs more power to drive it than a typical Nascar engine produces in total!The nitormethane fuel that is injected into these beasts is almost compressed into a SOLID rather than the liquid it starts out at.The engine only rotates a few hundred revolutions during it's run down the quarter mile, but it requires an entire rebuild / inspection after every run.Several of the clutch plates (and there's MANY of them) end up 'welded together' by the end of each pass.Assuming nothing major goes wrong, each pass costs several thousand dollars just in 'routine maintenance'.Given the sheer power produced by these engines, the crankshaft is intentionally 'skewed' to account for the amount that it 'twists' when under full power.It's totally INSANE (which is probably why I like it so much?)

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 13, 2013, 02:00:51 am
content: And if NASA were to design it, they would just put a rocket engine in the back and call it done...

author: Circlotron
author_rank: Super Contributor
timestamp: September 03, 2020, 01:25:53 pm
content: Quote from: TheRevva on May 09, 2013, 03:39:41 amVery old thread that caught my eye.I'd say you are talking about the MSD ProMag 44. They are indeed rated at 44 amps output, but this feeds into a 35:1 step-up ignition coil to get the 50KV and as a consequence the spark plugs get fed "only..." 1.25 amps.