Fitting a rev counter... to a strimmer? How?

[methoddan]

Hello,

I thought it would be fun to stick a tachometer onto my strimmer, mixing my two hobbies somewhat. Stick it in the middle on the bracket where the cow bars are mounted. A proper round dial tachometer that is, not one of these little battery LED ones.
You can buy cheap rev counters that go up to 13 or even 16000 rpm compatible with single cylinder, 2-stroke engines - finding one is no problem.

What I'm wondering is, how to connect it up? I know more about the mechanical side of engines than the electronic side, but I imagine in a strimmer (it's a Stihl FS450 if that's of any relevance) all you'd have to pick revs up from is the HT lead.

Here's a wiring diagram for said cheap rev counter: http://i.ebayimg.com/00/s/MTAwMVgxMDAx/z/DJEAAOSwf...
Forget the light wires, I'll cut them off. But what about the other three? Where would they connect?

Thanks in advance for any help,
- Dan

[andyiley]

It seems fairly simple really, although poor translation has made it less so, green + black - black/yellow to impulse signal, whether that would be from a sender, the coil, or the HT side should be explained in the gumph from the supplier.

[Athlon]

Em is it not green to ground and black to +ve? Yellow strip to negative of the coil but I guess a mower may have a magneto which may make it tricky..

[methoddan]

Hi guys,

Thanks for the replies. It's simple when you know it I guess!
I'm guessing that yes, green goes to earth, yellow to the coil makes complete sense... but what about the black wire? Also to the coil? In the same place? Picture says 'positive' yet I'm told to put the yellow wire to the negative side of the coil.

Also, out of curiosity and not completely understanding how these things work without batteries; where would the power come from? There's no obvious flywheel that I can see - it's a strimmer.

If it won't work, it won't work. But it'd be pretty funky if it did.