You could (should) start new thread, especially if you want to appoint another Solution
I find this project interesting, on behalf of what I work on, a MCU based 'computer' for bicycles with dynamo. From counting the Hz from the dynamo I can calculate the speed.
Once again I tender apologies to anyone who may have spent time looking at my problem.
Turns out I had omitted a decoupling capacitor at the input to the comparator, and this caused a doubling of comparator steps at low input frequency. The issue was not visible while using the cheap ebay digital 'CRO', but was apparent when i attached a real CRO. Then I saw the missing cap. Installation of cap fixed it.
Thanks and apologies.
mike
" You could (should) start new thread, especially if you want to appoint another Solution"
I'm sure you're right, but i struggle with the site to find where to post a new query, or even how to post a new query.
However, for your dynamo project, either the measurement of the period or the counting of cycles would work, which is best would depend on the dynamo output. Good luck, the fine and far more knowledgable folk on this forum will doubless be able to help
Well you managed to start this thread..?
Coudn't remember how - guess I'm just a bit thick! 
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