Reading RPM of motorcycle's engine

[huu_tri0101]

I want to measure rpm of motorcycle's engine. The engine is 4 stroke, one cylinder. So in one cycle, the crankshaft rolls 2 revolutions, the spark plug sparks 1 time. So I need to count time the spark plug sparks in one second to measure rpm.
To count time the spark plug sparks, I use the hall sensor place near motorcycle spark plug wire, then use an op-am to compare the Voltage that hall sensor gives back with the V ref. Then read the result form the op-am by the MCU.
Is that a good idea? Any suggest ?
Thank you guys. Sorry for my English.

[Kremmen]

A hall sensor is sensitive to a magnetic field so not the best possible one to sense the electric field around the HV spark cable. A better solution would be to make a pickup coil from a few turns of wire around the spark cable.
Google will tell you more. Some infor e.g. here: http://www.eng-tips.com/viewthread.cfm?qid=320604

[KJDS]

Pick up coil feeding a schmidt trigger is the way to go.

[A Hellene]

If you do not know EXACTLY what you are doing, DO NOT touch the pick-up coil. There are better ways of reading the engine speed.

Additionally, almost all of the single-cylinder motorcycle engines ignition controllers utilise the so-called "wasted spark" ignition system, since they are sensing the crankshaft rotational position and speed (where there is one spark for every crankshaft cycle); except, of course, if they have their sensors on the camshaft.

The answer you might be looking for is at the first link, above, by reading the charging system output frequency.


-George

[AndyC_772]

If your bike has electronic ignition, then there may already be a perfectly good signal you can tap into at the back of the clocks, feeding the rev counter. My bike has a nice, clean 12V square wave available which I've used successfully, and there's a similar signal for the speedometer on a nearby pin.

If you place a pick-up coil near to one of the ignition leads, then you should be able to pick up a strong signal quite easily - but it's worth checking it with a scope to see what it looks like before designing any kind of interface. Expect the ignition pulses to be surprisingly high in amplitude and very brief in duration.