Martin,

> You could replace the ADC0804 with a PIC and perform filtering in
> the PIC firmware

As John suggested, the prop sensor signal might not go through F-V
conversion, but be presented to the 8051 directly as digital. However,
I did trace the signal to one of the 4069 switch ICs. My goal for the
short time I had was to find out if a steady frequency gave a steady
display. Just for curiosity I did poke around in the circuit afterwards

I already have a PIC in there which I can add s/w to. His prop puts
out 2 pulses/rev, the VM expects 4, so mine doubles the frequency

> Option 2:
> Cut the least significant bit wire which leaves the ADC0804

I put it to him and he'd prefer not as there are other displays on the
cable. Given the age (1989) of it, the tolerances of components, and
the percentage error of 20rpm at 2500rpm, it's not that bad really.
And I was pleasantly surprised at the build quality too. Very nice

Carl,

> Have you tried contacting Vision? The organizations that got started=20
> catering to the homebuilt plane market were generally quite helpful.
> http://www.visionmicrosystems.com/faq/faq_index.html

Thanks, I did have a quick look initially but didn't really follow it up

> 5: Generating a signal on the bench, are you able to duplicate the issue=
=20
> to isolate the the panel unit?

Yes. If I feed it with 50Hz (transformer secondary) the display will
flicker more or less evenly between 740 and 760 rpm which leads me
to suspecting the 8051 s/w. At 100Hz input it is steady at 1500rpm,
ditto at 200Hz/3000rpm. Deviating from /20 gets it flickering

(It expects 4 pulses/rev, so 50Hz =3D 12.5rps =3D 750rpm)

> The RPM might be off by a factor of 2 or 4, ??

I'm fairly sure there's nothing wrong with the signal path

John,

> Note that, to measure RPM with a resolution of 20, you have to
> measure at least 3 seconds, if the speed is actually taken from the
> axle

My other contribution to his plane is the variable prop controller. I
count signal pulses and divide by time to get rpm, on a longer time
scale than this unit does

> - enlarge the gate time, then divide. But then the response time will
> be unreasonable - the time from changeing throttle to readout

Although a plane has generally more inertia than a car for example.
Most of the time is in cruise, not changing gear to go around corners
or laying rubber in front of Smokey ;-)

> Though I like precision too, 2500 +/- 20 looks quite precise to me

I believe the display is showing, as best it can, the true prop speed
on average

> Jumping LCD/LED displays are the curse of modern technology -=20
> nobody cared when the old mechanical RPM needle was nearly
> 100 RPM wide...

The owner has thought of a meter but finding one to match the others
might be a problem. The VM display has 100rpm graduations around
the circumference of the dial, which incidentally don't flicker on a 100
boundary when the centre digits do, leading to further speculation
about the s/w

Thanks for all the replies

Joe
--=20
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