Hello again Bill Bob, Groily and others. A few points have been raised so here is my latest 'tuppence worth', (to quote Bill).
First, although flattered to be accorded the title of 'electrical guru', I am afraid this is an overstatement of great magnitude. I was a Marine Engineer by background (so am really more comfortable with big diesels and steam plant than electronics). The little bit I have picked up comes purely from association with others I have worked with (and its over 20 years since I did any real engineering anyway).
So in my role as student rather than teacher, I like everyone else am learning all the time (and I have learned a lot since coming onto this forum, thanks to all of you out there).
Second point is Bob's Tacho idea. Bearing in mind reservations about my credentials, as stated above, I think it should be possible to do this quite easily and cheaply working with the low voltage side of the Thorspark. If it is producing a short 12 volt pulse, once every engine rev, it should be possible to pick this up and compare it against an electronic time base. Dividing one by the other and outputting as a digital (or maybe even an analogue) signal, suitably scaled, would give speed. I think there will be chips available at low cost to do this and we have a 12 volt supply readily at hand to drive the thing (power consumption would be negligible). You could read off with your voltage tester or rig up a permanent display. Sounds really promising Bob! There might even be something on the market already made up to do most of this for us.
Related to the above, I had noted from Bob's earlier posts that his bike is still work in progress but the test I was suggesting can be performed on the bench anyway as all you need to do is connect a test lamp (or use the volt tester) and see if it does indeed turn on then off as you slowly turn the magnet past the spot on the red pick up unit. Its the same idea as testing for a contact breaker opening except there is a much shorter dwell period.
Now to Bill's point. As you say Bill, you can get devices that pick up speed using an inductive pickup off the HT side but as you also say, these are expensive so if we could do it with what is already available, courtesy of Thorspark, on the LT side, that may be simpler and cheaper. Come to think of it, its another potential market opportunity for Thorspark to exploit as a 'bolt-on'?
Finally to Groily. Yes, I miss my slotted mag already as, if I can ever get the bloody thing started (I am going back to my other thread about this soon) I think I may want to retard a bit from the current 3/8. And it used to be oh so easy before when I had the slotted flange.
I had not really thought much about the issue of where the optimum magnetic flux comes on a mag and the potential problems with V twins, until you alerted me to this and its a good point that you make. I have very limited practical experience with mags but from a purely theoretical perspective, I would expect the flux to build up following a nominally sinusoidal profile so ideally we try to catch it (i.e. open the points) at exactly 90 degrees and 180 degrees. However, if we look at a Sine wave, it only drops by about +/-2% either side of 90 degrees (i.e. Sine 80 and Sine 100 = 0.985). So it might be more forgiving than may at first be apparent. Even if out by 25 degrees, Sine 65 = 0.9 so its only lost 10%. I wonder how these speculations match with reality?
Thanks again to you all
Alan