Re: ATM beginner projects

[Dwight Elvey <elvey@hal.com>]

"William A. Morgan" <sass5@erols.com> wrote:
> 1. Can the a/c motor like the one I described work with a rheostat, say a
> house
> hold dimmer switch to slow or vary the rpm's? 

Hi Bill
 No, in order to make a variable speed motor, you'll need
what is called a AC/DC motor. Most of these achieve variable
speed by controlling the current to the field windings.
Some lighter ones use pulse width modulation like the
SCR light dimmers.
Surprisingly to many, the motors speed goes up as less
current goes through the field windings. You can tell
if it is one of these type motors because they have brushes
and a commutator. The motors used in things like skill saws
and hand drills are of this type.

> 2. My understanding of the slit-less foucault tester is that you want to get
as
> close to the blade as possible, right? My idea  is to build  support that is
> 1/4
> inch thick or less for the test light and a 1/8 inch view hole with blade. 

 If you understand what is need, it will make more sense. You need
to have the edge unfocused such that its shadow extends the full
width of the mirror. Hold an edge up to your eye an place your
mirror at the about correct distance. You'll see that the shadow
of the edge needs to be quite close to get the desired effect.
 I like to have the edge completely free of anything on the
edge side. If I was to do it, I'd put a notch on the edge of the
board such that my nose ( a little to big ) would clear the
board. I don't think you'll find that you'll need any sighting
aids. In fact, having the edge open helps a little while
aligning the mirror. The LED's image has to be well aligned
with the edge vertically before anything else. Pulling one's
head back a little and adjusting the height of the tester
or mirror is a lot easier if you can clearly associate the
LED image to the knife edge. The LED's light will only be
as big in height at the knife edge as the original
LED was, assuming the moving source at the same distance.
This is because the mirror is only at 1X magnification. This
is also why you need to use one of the jumbo LEDs. The wider
the easier to use. Also, you might consider using a prism as
was mentioned by one of the others to get more clearance to
mount the LED, otherwise the LED's leads will be in your cheek
bone. Prisms from old binoculars would work great here. This
has the added advantage that you can move the LED farther away
from the source edge and minimize the effects of irregular
light intensities across the LED face, since they will be
more out of focus. Of course, the height of the image at
the knife edge will be a little smaller.
Dwight