Mike Sims wrote:
> Just finished a homebrew wattmeter and would like to make a non-linear
> meter
> scale. Also, have an amplifier and would like to redo the meter
> scales to
> directly display the measured values.
>
> Not interested in doing it free hand as when I do it they look like
> they
> were made with crayons using large lined paper.
>
> Can anyone describe how to make meter scales (faces) using programs
> that are
> relatively available on the home computer? The term "relative
> available"
> means something like the MS products, Draw, Word Perfect, etc. I've
> heard
> of making meter scales with various CAD packages. However, I don't
> have
> access to any of them.
I've done this using CorelDraw (version 5 I think), rather than
anyfreebie software, but my experiences might be helpful. I should state
its often possible to get early versions of Coreldraw for free with
computer magazines, or very cheap.
1) Take the meter apart carefully. Ensure there are no bits of metal
around, which wil get attracted to the meter by the magnet.
2) Measure the original scale - in particular the radius from the centre
to the scale, and the length of the scale (in mm)
3) Determine the length of the scale in degrees. Its standard
trigonometry, from data gathered in (1) above. Most are 90 or 100 deg in
arc.
3) Calculate the position of the new scales. I've done them for power
and swr, both of which are non-linear. However, since diodes have both
linear and non-linear (square law) regions, you can go too far with
just theory. You need to check them against something thats accurate. I
wrote a simple program that assumed a sqaure-law and conveted from
angular position (degrees) to power in Watts.
4) I have used different colours on some of my meter scales - green for
okay regions, yellow for getting too big/too small and red for
dangerous. An anode voltage meter for instance had a red region below
1kV (where scrren damage could result), a green region, then yellow and
red. Power meters have similar colour scales - handy if someone else
uses equipment.
5) One problem I have found is that some scales, printed with an HP
bubblejet printer, bleed over a year or so, so all the colours run a
little. Some scales did this badly, some not at all. Some were printed
on normal 80gramm/cm^2 photocopy paper, some on the expensive HP
bubblejet paper. Unfortunately, I am unable to determine if the colours
runs were only on one type of paper, but a friend who is more into
graphics than me, suggested that the more expensive paper would be more
colour stable. I should add the meters were placed in my gararge, where
the temperature changes over a year are enormous.
Dave Kirkby G8WRB
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