RPM sensor for gas motor

John Bond

Hi Guys

I would appreciate your help.

I'm trying to measure RPM off the plug lead of a gas (petrol) motor. The idea is to have a clip-on sensor that fits over a lead to one of the spark plugs and gives a suitable signal. Reliability is a bit important. Being able to fit and remove the device quickly and easily using unskilled people is very important hence the use of a plug lead sensor.

My searching has not turned up much (plenty on connecting to the low tension side or using "hall effect").

I found the following extract of a circuit that is used to trigger a car strobe light.·I'll obviously need to filter the spark "rebound".

QUESTION: -·Do any of you have a better circuit? If not, how·can I improve this one.

Kind regards from the bottom of the world
John Bond

Post Edited By Moderator (Bean (Hitt Consulting)) : 7/18/2007 12:26:13 PM GMT

PJAllen

I think that you may find that a coil is, perhaps,·over-complicating.· An op-amp (or maybe·your transistor) with an input connected to a length of wire as·an antenna·may well·get "signal" enough from the arcing-&-sparking that you could then further amplify and/or condition and make use of.

Update -- Careful around plug leads, when they get old and start to rot out, you could more than couple in the signal, you might draw an arc to your project.· So, better proximity than contact (IMHO.)

Post Edited (PJ Allen) : 7/18/2007 8:40:48 PM GMT

MikeS

I have one of these.

http://www.tinytach.com/tinytach/gasoline.php

You only have to wrap the pickup wire 3 times around the spark plug wire.



MikeS

Capt. Quirk

Look closer at the Melexis 90217 hall sensor data sheet It shows how to do it with a slotted ferrite ring and the hall sensor mounted inside the slot and your spark plug wire going through the rings core.

Also I have seen several applications, including automotive OEM's that used PJ Allen's suggestion. Although the OEM's would normally rap a wire around a ferrite ring and run the plug wire through the ring.

If it is just a piece of equipment, "Tiny Tach" sells a combination tach and hr. meter that is self powered and uses the wire rapped around the plug wire for about $40.00.

John Bond

Thanks Guys

Hey Capt Quirk, that "Application Notes" on hall effect is amazing. I've used them and known a fair amount about them but this level of detail...

Regards
John

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