AOH :: STATDRUM.TXT|
Description of a rotating disk device that makes E-Field visible.
ROTATING DISK DEVICE MAKES E-FIELD VISIBLE.....(c)1994 William Beaty
I built a prototype e-field visualizer based on a rotating plastic drum. Here
are suggestions for a better version, based on my experiences.
The spinning motion does not generate charge separation, but if anything
touches the spinning device (like clothing, rabbit fur wands, etc.) the
plastic parts become highly charged, creating strange, distorted fields. The
solution: build the device out of metal.
Rather than a rotating arm, a rotating disk with flush-mounted parts should be
much safer and easy to balance. Either that or a rotating drum shape. I think
the flat display of the disk is much better for classroom demos, while a
vertical-axis drum would be better for a museum exhibit.
The mechanically-swept LEDs are not bright enough to be easily seen. The metal
disk needs to be painted black, and the equivalent of SEVERAL rotating arms
need to be placed upon its surface. This also allows you to spin the disk
quite slowly and still have the LEDs scan fast enough to create the patches of
color. On the outer perimiter of the disk, there need to be a number of bright
LEDs at each radial distance. Close in, the LEDs can be fewer and dimmer.
There must be an optimum pattern for this which gives a uniform patch of LED
light. And rather than use red/green LEDs, individual superbright red and
green units can be placed adjacent to each other at the same radial distance,
and will blend to yellow while providing much higher output than an R/G LED
would. Also, flat top wide view LEDs should work much better than standard
I found that black anodized decorative aluminum is conductive, unlike black
spray paint, so that's what I would use for the disk surface.
The LEDs should be flush mounted or even recessed. Socket cap 4-40 screws in
black, mounted in nylon transistor-mount washers, make excellent little
antennas. And pre-built carbon brush holders can be had from a couple of
places in the Thomas Register.
Electrically this device is an AC system, which makes everything a whole lot
easier. Quad op amps would work, and FET-input versions might even be
When highly charged objects are held near the device, the antenna potential
can exceed the limits of the input electronics, and this causes a permanent
charging of the inputs. But since this is an AC system, a resistor value can
be chosen which leaks the charge away fast. Also, if a capacitor is placed
across the antenna and ground (a tiny value!), it acts as the second leg of a
capacitive voltage divider. The sensitivity goes way down, so it takes a lot
more voltage to overload the inputs. The gain of the amps can then be cranked
up to compensate.
Once this device is complete, you can install infrared, hall-effect,
microwave, UV, microphone, ultrasonic, etc. sensors IN THE SAME DEVICE and end
up with a scanned phosphor screen which responds to numerous fields. Getting
it working with sound, then placing an acoustic lens in front, would give you
a passive sound camera!!! What would you see?
.....................uuuu / oo \ uuuu........,......................
William Beaty voice:206-781-3320 bbs:206-789-0775 cserv:71241,3623
EE/Programmer/Science exhibit designer . .http://www.eskimo.com/~billb
Seattle, WA 98117 firstname.lastname@example.org . . . SCIENCE EXHIBITS web page
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