fitting a cheap ebay tacho

[WD Forte]

So I decided I wanted a tacho on the scoot
so I squandered nearly 12 quid for this one from Ebay.
12 bloody quid! I could have bought nearly a gallon of Diamond White cider for that
but we have to make these sacrifices in the name of science and all that I spose.

Too much faff and too little room to easily mount in on the dash panel
so I'm going to mount it on the bar cover with the cables following the others
down into the front fairing and had to patch in longer cables and while I was at it
I decided to test it on the bench to see if it worked and how accurate it was.

The white back light worked well enough for a cheapy thing.
I dug out a wee signal generator and got it to put out a square wave at
various frequencies to emulate engine speeds up to 10,000 rpm
16hz for 1000 rpm, up to 166 hz for 10,000 rpm
I checked the frequency output on a scope and it more or less corresponded to
the signal generator display within 1 hertz so I'll assume it's near as
and mapped out what the tacho showed against the signal it got.
rpm hz tacho reading
1000 = 16 hz a smidge over 1000 rpm
2000 = 33.33 hz just over 2000 rpm
3000 = 50h z just over 3000 rpm
4000 = 66.66 hz just over 4000 rpm
5000 = 83.33 hz just over 5000 rpm
6000 = 100 hz a smidge over 6000 rpm
7000 = 116.66 hz just over 7000 rpm
8000 = 133.33 hz around 8300 rpm
9000 = 150 hz around 9600
10000 = 166 hz around 10700

So, for twelvety pounds it's as one would expect for the price really
but at least I have an idea how it will behave.
I'll probably be using at the 6000/7000 range most of the time,
not even sure the poor wee engine will go up to 10k.

I'll check its performance again with the scope when its fitted and
tapped into the ignition pulser wire to see how it behaves.
Pulsers often put out a rough AC signal so I may run the signal pulses via at least
a diode to condition it a bit, but will find out when I try it IRL.
Pics

https://imgur.com/Lrihv0L.jpg

https://imgur.com/XuhdkbF.jpg

https://imgur.com/EEkYBSV.jpg

16hz = 1000 rpm around engine idle speed

https://imgur.com/AjNq7bx.jpg
____________________
bikers smell of wee

[Nobby the Bastard]

Seems to be reacting about right (7000 rpm is 116.6 rps)
____________________
trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Sprint ST 1050

[WD Forte]

Well the tacho's in and working
At first just strapping the tacho input to the pulser appeared to work but proved to be erratic and at higher revs it went a bit mad with the needle going up to 12,000 rpm at
an actual 7000 rpm -ish

The waveform from the pulser is pretty typical.

https://imgur.com/KKQBQ2i.jpg


I found the tacho was working from the falling edge of the pulses, so put a diode in
reverse series to block positive voltages and this settled it down.

https://imgur.com/Gstg02e.jpg


It's pretty much as good as its going to get now.
The diplayed rpm corresponds with frequency of the pulses within 100rpm or so.
from tickover to 8000 rpm and that'll do me.

https://imgur.com/syYsnyd.jpg


BTW that's not an oil pressure light on the console.
Its an annoying service light supposed to tell owners when it's time to change the oil but its a bit random as it doesn't need an oil change, the battery has been out a while.
Cant remember how to reset the wee fecker, will have to RTFM I spose.
____________________
bikers smell of wee