Spark plug wire inductive pickup for RPM

[RobF]

So recently I've seen several devices that have a pretty neat way of interfacing with most modern day combustion engines. Instead of complicating the installation of a device with having to find an RPM pickup to splice into(coil trigger, engine position sensor, etc) several manufacturers are suppling their devices with a one wire inductive pickup. They instruct the user to wrap the end of the wire around the spark plug wire several times (7 I think). Specifically, many hour meters use this style. This is very neat if you can actually determine RPM from it (some do some don't). I technically don't need to read RPM (which is a lot easier) but I would like to have the capability in the future.

I could wrap some wire around a spark plug wire myself and do the test to capture the waveforms but I figured I'd stop by here first. Has anyone successfully built the analog circuitry to condition this signal into something TTL compatible?

Here is an example:

http://www.tinytach.com/tinytach/installation_gas.html

Thanks guys

 

[Skogsgurra]

You do not need to wrap the pick up wire around the ignition cable. The signal pick up is capacitive, so it is OK to just slip a tube over the cable and put the pick-up wire inside that tube for a few inches..

Be careful not to put your signal wire close to other ignition cables - it will easily pick up enough signal to confuse your counter if you do.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[GregLocock]

I'd add that looking at the 12V or 5V side of things is far more reliable, if you want a speed output.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[VEBill]

The most modern approach is to simply plug into the mandated OBD port and pull the required data from there. The primary disadvantage is that some cars have the OBD socket in a position that would leave the OBD cable dangling across the dash (trailing along next to the GPS power cord, and multiple satellite radio cables, etc. ;-) ).

Not sure if large trucks have OBD, but most of them are diesel and wouldn't have spark plugs wires either.

Some modern cars have the coils right on the spark plugs, so no distributor and no spark plug wires.

 

[itsmoked]

Dang I typed this whole thing and must've not posted it.

I used LMC550s. The first was setup as a unity buffer. The input was DC biased to the center. The input was capacitive just like the above examples.

The buffer was followed by a comparator that just squarewaves what the buffer puts out. Then you just look at the squarewaves.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Yes you did post it Keith. I was very confused by your reply in another forum. But it was yesterday and I don't remember which one.
LOL

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

LOL! I stopped for a break from my tedious work and checked ET then got, called several times. I got back to my comp and decided to close all unnecessary windows. I had about 4 ET instances going. I must've started my answer in the wrong one.

Hopefully someone axed it for me.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Thanks for the heads up Bill. I found it in some Magnetics {Lm) thread.

Keith Cress
kcress -

http://www.flaminsystems.com