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Capacitive rpm meter

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senso:
Good night, here I am seeking for your advice again  :-[

I 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?

TheRevva:
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.

peter.mitchell:
Just get an extruded aluminium project box and powdercoat it

SeanB:
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.

senso:
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 tesla ;)

When 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  :o

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