sounds like a one cylinder engine, now is it a two or four stroke engine? four strokes fire once every other rev, two strokes fire every rev.

Not necessarily, Depends where the points are. Briggs and Tecumseh, and the like are waste spark, the points on the crank.

 

Great ! So now I have to find more about the engine to determine the spark timing and rotation ..eh !

 

Great ! So now I have to find more about the engine to determine the spark timing and rotation ..eh !

Why? Your just wanting a pulse from each time the plug/coil fires. It's not like you are making a "real" tachometer. And rotation doesn't mater at all.

 

It's for fancy show off.

So I guess it's time to draw up something. I only want all the led's to light when the throttle is maxed. Don't need no exact rpm.

Lesse ! How many turns would you guys use to begin with around the spark wire ?

 

Great ! So now I have to find more about the engine to determine the spark timing and rotation ..eh !

You don't have to find out much - most 4-strokes have the points cam on the camshaft which turns 1/2 the crankshaft speed.

IIRC: 360 deg twins have 2 points lobes and use the "wasted spark" system, 180 deg twins have 2 coils and 2 sets of points mounted 90 deg apart on the camshaft stator plate, operated by a single lobe.

Some 360 deg twins have a single lobe points cam on the end of the crankshaft.

 

It's for fancy show off.

So I guess it's time to draw up something. I only want all the led's to light when the throttle is maxed. Don't need no exact rpm.

Lesse ! How many turns would you guys use to begin with around the spark wire ?

You'd probably need a lot of turns round the HT lead, its more the length of winding than the number of turns since the wire is at the wrong axis.

I'd go for putting 20 - 30 turns on a ferrite toroid and slip that on the HT lead. You'd want to pretty much pot it with hot melt glue to keep your secondary dry and stop vibration chaffing the HT insulation.

You could also adopt the clip on sensor from a broken timing strobe.

Whatever you use for an input amplifier - you're sensor is a current transformer with pulses of pretty high peak instantaneous current in the primary - the secondary will produce alarming peak voltage pulsed. You'll need clamp diodes to protect the input circuit.

 

I was thinking of winding some coil on a cylinder and pass the spark wire through the cylinder. But I guess it won't work.

So. I need to find a ferrite ring core, wind some wire on the core and pass the spark wore thru the core, right ?

Should I wind the coil covering the whole core or just part of it ?
I believe the later is what you use to sense current through high voltage wires, like those used in electrical panels and high power AC motor terminals.

I know I would need to clamp the input voltage amp or integrator to protect the input from spikes.

What do you mean by "wrong axis"

 

I was thinking of winding some coil on a cylinder and pass the spark wire through the cylinder. But I guess it won't work.

So. I need to find a ferrite ring core, wind some wire on the core and pass the spark wore thru the core, right ?

"

Ferrite rings aren't that hard to come by, most things that run off a SMPSU do. You want one with a fairly small mass, but still big enough for your winding andstill have room to pass the HT lead through.

They're often fitted on the bundles of wires from a PC motherboard to the LEDs/switches on the front panel.

Older motherboards tended to have bigger toroids for the onboard buck regulators - the HT lead just might slip through if you take a few turns off.

 

I have ferrite core of several types.
After all I run a workshop and have all sorta consumer junks.

I will try tht method soon.

 

I'd use the smallest ferrite toroid you have that the secondary ignition wire (spark plug wire) will fit through with 10-20 turns on the toroid. Large toroids are used to suppress RFI on wires going into/out of electronic enclosures; but you really don't want to take a whole lot of energy from that spark.

For testing, you could saw a toroid in half, and wind just 1/2 of it with the turns of wire. Glue the halves of the toroid in a wooden clothespin (the type with a wire spring hinge) so that you can easily clip it onto/remove it from the ignition wire. My grandfather did this for a pickup for a Heathkit tachometer that he built in the 60's for his outboard motor.

By the way, a modified clothespin makes a good holder for winding small toroids. Have a look here:
http://wa6dzs.blogspot.com/2009/12/toroid-clip-from-clothespin.html

 

Thanks Sgt.
Ur insight was just what I needed.
By the way. How are things, if you know what I mean.

 

Things are quite good, thanks - if very busy! It's 1chance's birthday tomorrow - and we had an ice storm last night, so we will be spending the rest of the weekend at home. I got her a new top-of-the-line laptop/tablet for Christmas, but it had Windows 8 installed, and immediately wanted to update to Win 8.1 - to keep an already-too-long story short, it sucked up almost all of our bandwidth allotment for the month in ONE DAY, and still wasn't updated! But the bandwidth counter resets tomorrow, so all will be good then...

 

All ive been using on my tacho is about 5 turns of wire around the ignition cable then cable ty it so it doesn't unwind.

 

All ive been using on my tacho is about 5 turns of wire around the ignition cable then cable ty it so it doesn't unwind.

U mean just wrap em around the spark plug cable?
How many turns and what Gauge ?

 

Pics of a tacho/hr meter on my generator. $12 unit of Ebay. You could also use an LM2917 tacho chip to drive LED bar/dot display. Pretty sure this chip will work with a few turns of wire around the coil wire. Just hook up type wire.

 

You might try electrostatic coupling.
Pick-up requirement reminded me of a project from another century.

A time base generator phase locked to fly-back circuit of tv.
At one time it was ultimate accuracy a hobbyist could make.
Published in QST magazine.

A short whip from the base of Q1 would lock-on to any tv within a couple feet.

Even though it's an entirely different freq., the simple circuit may still work.
Subbing the LM291?

 

Been looking every where for this, its a kit I acquired some yeas ago LED tacho. Will assemble it in the next few days, & ime sure it can be modified to do the job.

 

Hooked up to a single cylinder 2stoke, about 6turns around the plug lead. This worked with R4 bypassed,

 

@debe
You made the circuit in post #37 , right?

In post #38 I see one wire coming from pickup coil, I assume the second should be grounded but I do not see it in ur picture

 

Yes the circuit board needs an earth to the engine. Yes assembled the kit yesterday, it has been siting around un assembled for quite some years. Over the years ive used a lot of the LM2917 chip in analogue tachos for diesel engines using a Hall pickup.