In a message dated 20/10/98 17:12:03, bob.fairall@btinternet.com writes:
<< ..... until I go above about 5000 when
the analogue unit goes bananas and reads anything from 0 - 7000rpm. >>
I don't know what's in the analogue unit, but you have only to look at the
input waveform to see why any type can go bananas at high revs.
It is an inductive pickup which grows with revs. and the waveform has many
spurious pulses and noise between the real ones which will be picked up
sooner or later.
I built a digital version which behaved like this, but you can condition the
input signal to avoid the problem. The simplest is just to attentuate it
sufficiently to read correctly at say 6000, and you may then find it ok over
the required range, though there is a risk of drop out at low revs. There are
more elaborate solutions which it would be out of place to go into here.
Graham C. G-EMIN
|