ROTATING DISK DEVICE MAKES E-FIELDS OR SOUND VISIBLE
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 device consisted of a motorized rotating drum, with a closely-spaced row of sensors upon its surface. Each sensor had an amplifier which drove an LED, and the LED was placed adjacent to the sensor. When the drum was spinning, the row of LEDs formed a sort of raster pattern, like a cylindrical phosphor screen. When electrified objects were held near the drum, their e-fields affected the "phosphor screen" and caused bright or dark patches to appear within the uniform field of red light. For the sensors, see Charge Detector.

NOTES:

The spinning motion of the drum or disk 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.

A rotating disk with flush-mounted parts should be much safer and easy to balance than a rotating arm. Either that or use 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 spinning part could even be hand-cranked rather than motorized.

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 visible 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 (i.e. use constant spacing between LEDs?) And rather than use signle multicolor 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 a single R/G LED would. Also, for a rotating drum, flat top wide view LEDs should work much better than standard 20-degree LEDs.

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 for the slip-rings, pre-built carbon brush holders can be had from a couple of places in the Thomas Register. That, or make yourself a rotary transformer with a bundle of iron welding rods as the core.

Electrically this e-field device is an AC system, which makes everything a whole lot easier. Quad op amps would work, and FET-input versions might even be unnecessary.

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, IR phototransistors, UV sensors, microphones, ultrasonic, ZnS-coated scintillator photocells, etc., IN THE SAME DEVICE and end up with a scanned phosphor screen which responds to numerous fields. Dipoles and opamps microwave sensors would give you a "radio-wave phosphor panel," Get it working with sound, then place an acoustic lens in front, and you'd have a passive acoustic camera, an "eye" for sound waves!!! What would you see?