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Old 6th Aug 2022, 4:20 pm   #1
pentoad
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Default Using a dmm as a tachometer

I have a dmm with an HZ function, could I use it to measure RPM of a single cylinder petrol engine by wrapping a few turns of wire around the spark plug lead
to get a reading then divide the result by 60?
I have not tried the experiment as I was worried that despite electrical insulation evh could migrate into the dmm and destoy it
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Old 6th Aug 2022, 4:31 pm   #2
duncanlowe
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Default Re: Using a dmm as a tachometer

Can you get at the low tension side of the system? Often the stop switch simply grounds part of the system so when running it's not grounded, so in theory you could measure across the stop switch. Away from the HT stuff but might still be spikes.
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Old 6th Aug 2022, 4:36 pm   #3
Leon Crampin
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Default Re: Using a dmm as a tachometer

Hmm. I wouldn't risk a decent meter doing this. Remamber to ascertain if the engine is 2-stroke or 4-stroke and calculate accordingly.

Without straying into forbidden forum territory, you could more safely extract a speed signal from the primary side of the coil or magneto. Old engines use points, more modern units Hall sensors. High voltage spikes are still a risk though, especially on older systems with points. Mind your fingers - especially near rotating machinery.

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Old 6th Aug 2022, 5:32 pm   #4
G6Tanuki
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Default Re: Using a dmm as a tachometer

A normal DMM will be assuming something like a sinewave at its input, the waveforms from a loosely coupled input from a points/condenser classic Kettering ignition circuit come with loads of complications, which it is not competent to interpret.

In days past I built a bunch of digital revcounters from circuits in Practical Electronics. Elektor etc.. They never really worked well... 'points bounce' invariably caused misreading at high RPM, and we learned to divide the displayed RPM by two to compensate.
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Old 6th Aug 2022, 5:53 pm   #5
factory
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Default Re: Using a dmm as a tachometer

Probably easier to make an optical tachometer and yes the counter reading needs dividing by 60, unless you have one that will read in cycles per second. I have two vintage optical tachometer attachments, one valve (HP) & the other transistor based (Airmec).

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Old 6th Aug 2022, 6:28 pm   #6
pentoad
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Default Re: Using a dmm as a tachometer

Thank you to everbody who replied, so it's a resounding no!
I can buy one for a tenner, but I think I will only use it once.
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Old 6th Aug 2022, 6:52 pm   #7
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Default Re: Using a dmm as a tachometer

divide by 60?

In my experience you have to careful even with a digital frequency counter as they tend to work with nice waveforms and they detect crossing points (or offsets from 0) so noise or non-negative-going signals can be an issue.
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Old 6th Aug 2022, 9:27 pm   #8
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Default Re: Using a dmm as a tachometer

There are a few engine RPM apps for mobile phones using the mic - not sure if they work but worth a try.
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Old 6th Aug 2022, 9:54 pm   #9
factory
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Default Re: Using a dmm as a tachometer

The "cycles per second" in my previous post should read "cycles per minute" , I was thinking of the HP 5307A that has a CPM button.

Don't forget some reflective tape, if using an optical tacho.

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Old 7th Aug 2022, 7:41 am   #10
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Default Re: Using a dmm as a tachometer

Quote:
Originally Posted by Leon Crampin View Post
Hmm. I wouldn't risk a decent meter doing this. Remamber to ascertain if the engine is 2-stroke or 4-stroke and calculate accordingly.

Without straying into forbidden forum territory, you could more safely extract a speed signal from the primary side of the coil or magneto. Old engines use points, more modern units Hall sensors. High voltage spikes are still a risk though, especially on older systems with points. Mind your fingers - especially near rotating machinery.

Leon.

(Crossed with Duncan).
I once destroyed a cheap multimeter when checking continuity of the mag. on a lawn mower. All it took was a manual movement of the motor and the efficient mag. produced a nice voltage spike!
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Old 7th Aug 2022, 10:35 am   #11
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Default Re: Using a dmm as a tachometer

Optical tachometers are available below 15 (quality unknown) but it's best to weed out the non-uk sellers who try to conceal that they are thousands of miles away.
I guess if you buy one and someone subsequently borrows it and owes you a favour- then it's paid for itself.

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Old 8th Aug 2022, 11:47 am   #12
pentoad
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Default Re: Using a dmm as a tachometer

Quote:
Originally Posted by The Philpott View Post
Optical tachometers are available below 15 (quality unknown) but it's best to weed out the non-uk sellers who try to conceal that they are thousands of miles away.
I guess if you buy one and someone subsequently borrows it and owes you a favour- then it's paid for itself.

Dave
I won't be able to use an optical type on this engine as there nothing visibly rotating to point the thing at as everything is enclosed.
There are devices which a wire wrapped around the spark plug lead is used to detect the rpm - not sure how accurate the are
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Old 8th Aug 2022, 12:48 pm   #13
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Default Re: Using a dmm as a tachometer

The classic way to get a RPM signal for a revcounter was to use a current transformer - typically a couple of turns of the ignition coil feed or its magneto equivalent. This would produce quite enough drive for a Germanium transistor pulse counter, and being only concerned with current not voltage it wouldn't die if a HT lead fell off...
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Old 10th Aug 2022, 6:12 pm   #14
G6fylneil
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Default Re: Using a dmm as a tachometer

I've got a '70s timing light with an analogue RPM meter that uses a split current transformer pickup.
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Old 13th Aug 2022, 5:51 am   #15
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Default Re: Using a dmm as a tachometer

Is this a modern car with OBD2 port? A Bluetooth dongle for the port and an app such as Torque or Carscanner will give you loads of data
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Old 13th Aug 2022, 5:55 am   #16
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Default Re: Using a dmm as a tachometer

Sorry re read original post, I presume this is a very simple engine! If it's a motorcycle or whatever with an alternator would you be able to get a clean sine wave off that?
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Old 14th Aug 2022, 6:51 pm   #17
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Default Re: Using a dmm as a tachometer

Quote:
and we learned to divide the displayed RPM by two to compensate
You do realise that many single cylinder engines spark each time the piston is at the top (because it doesn't matter).
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Old 15th Aug 2022, 3:32 pm   #18
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Default Re: Using a dmm as a tachometer

'Wasted spark' operation. Had forgotten about that. Also crops up in the modern era.
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Old 17th Aug 2022, 6:06 pm   #19
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Default Re: Using a dmm as a tachometer

Some one might have a copy of an old practical ??( circa 70's) where a design for a decent CDI ignition system was produced. In the article the drive for the tacho was derived from a 555. I built one of these and fed the output to a small meter, which was designed as a false 0 meter, meaning that the output was shown as times something. Many years later I've not got a copy of the circuit as in the 80's my rescue dog decided to use my mags and copies as a bed for her pups. But theinfo might help.
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Old 18th Aug 2022, 6:31 am   #20
Electronpusher0
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Default Re: Using a dmm as a tachometer

Practical Electronics published the Scorpio electronic ignition. I built it and it worked well.
The Mk2 version was published in the Mar 74 edition.

https://worldradiohistory.com/UK/Pra...cs-1974-03.pdf

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