Plasma Saber

Banish the force of darkness with this glowing-light foil and learn about portable high-voltage power sources.

At any given moment, half our planet is engulfed in the dark side. While there may not be any evil forces trying to conquer or take us over by force, here's a fun and exciting way to defeat the darkness ...a way to wave cohesive light before you as you move through the night.

While it may not turn you into a mystical warrior, the Plasma Saber described in this article is an edu- catlonal and enjoyable project. As you build this handheld high-voltage source, you'll be learning by doing. As you swing the glowing neon blade of your Saber, you'll be the envy of everyone on your block, be they sci-fi fans or not.

Circuit Operation. A schematic diagram of the Plasma Saber is shown In Fig. 1. The circuit is comprised of three transistors (Q1-Q3), a custom-wound transformer (T1), and a few support components. Together those components form a high-fre- quency, high-voltage power supply that's capable of driving a neon- plasma display, discharge tube (NE1).

Note that the circuit contains four batteries: A pair of parallel-connected 9-volt batteries (B1 and B2) are wired in series with a couple of series-connected 1.5-volt AA batteries (B3 and B4). Batteries B3 and 84 are required only when a red neon tube is used for NE1; otherwise, those batteries must be eliminated during construction. Power from the batteries Is applied directly to the collector of Q2 and, via the R1/C1 parallel combination, to the base of Q1. As long as the touch contacts remain unbridged, a positive voltage is applied to the base of Q1, a PN2907 PNP bipolar transistor, keeping Q1 biased at cutoff.

However, when the two touch contacts are bridged (by the user through hand contact), the bias voltage appearing at the base of Ql is pulled low. (Recall that in order for a bipolar PNP transistor to conduct, its collector must be more negative than its base, and its base more negative than its emitter.) Note: The amount of bias applied to the base of Ql depends on the user's skin resistance.

With Ql's base pulled low, it begins to conduct, feeding a variable current ramp to the base of Q2 (an NPN unit whose turn-on criteria is the opposite to that of the PNP unit), That causes Q2 to turn on, feeding a ramp current to the collector of Q3-which is configured as a modified Hartley oscillaator-through the primary winding of transformer Tl.

The Hartley oscillator-used rather extensively in AM and FM radio receivers-is a form of variable-frequency oscillator (VFO), whose operating frequency is usually determined by a parallel combination of inductance and capacitance (tank circuit) in the feedback loop. However, our version of the Hartley oscillator uses an auxiliary winding of Tl in conjunction with R4 to form a properly phased feedback network, through which a drive signal is delivered to the base of Q3. That signal causes the Hartley to oscillate at a frequency-determined by the resonant frequency of the inductance (the auxiliary winding of T1) in the feed- back loop-of approximately 200 kHz. The rising and collapsing field created in the primary windings of T1 generates an alternating high- voltage output (approximately 6kV) In T1 's secondary winding that is fed to NE1, causing it to glow.

No on/off switch is necessary since total power is controlled by the user's skin contact. Note: A dry hand may require a tighter grip where a damp hand requires only a light touch to achieve full plasma ignition. Capacitor C 1 is included in the circuit to bypass any external signals that could potentially cause premature operation, while R2 sets the sensitivity range of the circuit.

Electronic Construction. The majority of the electronic components for the Plasma Saber were assembled on a small printed-circuit board, measuring 23/4 by 5/8 Inches. A full-size template of that printed-circuit foil pattern is shown in Fig. 2 for those who prefer to etch their own printed-circuit boards. For those not so inclined, a complete kit of parts, as well as selected Plasma Saber components and pre-assembled units, is available from the supplier listed in the Parts List. Table 11ists kit, selected components, and assembled unit prices (contact the supplier for further information).

For ease of construction, the Plasma Saber was assembled in two parts-the display and power sections. The Saber was assembled so that should the plasma discharge tube become broken or damaged, it can easily be replaced. The two-section assembly scheme also allows the plasma tube to be replaced should the builder decide to change the display colors (recall the need for batteries 83 and 84 if a red tube is used). The display section of the device can consist of a 12- to 36-inch length of small-diameter neon or other gas tube.

Regardless of which route you take, once you've obtained all of the parts listed in the Parts List, construction can begin. Assemble the project guided by the parts-placement diagram show~ in Fig. 3. Note that If you are building the unit from a kit, some of the component values may vary from that specified. That is acceptable, since all of the components used in the Saber have a tolerance of 10 to 20%, unless otherwise noted. Begin board assembly by installing all the passive components the exception of the transformer) on the printed-circuit board first, followed by the solid-state devices. Note: All board-mounted components should be mounted slightly elevated (about 1/8 to 1/4 Inch) above the board surface. Be sure to observe the proper orientation of the polarized components-C2 and Q1-Q3. After each component is soldered in place, cut away any excess lead length.

Now we come to the installation of T1 .If you've purchased a kit of parts or the transformer (only) from the kit supplier listed in the Parts List, install the unit as outlined in this paragraph. If, on the other hand, you intend to wind your own transformer, follow the coil winding instructions given in the next paragraph and mount the unit as out- lined here. Secure the body of T1 (a tubular custom-made high-frequency unit) to the board with tape, and then connect the transformer to the appropriate circuit board pads using short lengths of buss wire, as shown in Fig. 3. Note from the pinout diagram (shown below T1 in the parts-placement diagram) that T1 is mounted with pins 1 and 5 toward the surface of the board.

Figure 4 gives details for custom winding your own transformer. The transformer was fabricated by bifilar winding 10 turns of 26 AWG and 10 turns of 28 AWG wire on a tubular type TC75D-I coil form, leaving a little extra wire length for connectIon to the terminal pins of the coil form. After winding them, secure the coils in place with a piece of electrical tape. Using a multi-meter, identify the ends of the 26 AWG wire and connect them to the coil form (which has numbered terminal pins), as shown In Fig. 4. Follow that by winding 1350 turns of 38 AWG wire for the secondary winding on top of the bifilar-wound coils. Connect the bottom end of the secondary winding to pin 5 of the coil form, mating It with the lower end of the second (auxiliary) primary winding. Note: The other end of the secondary winding does not connect to a coil-form pin, but instead connects to an insulated wire lead that is brought out through the rear (stud) end of the coil. Cover the entire assembly in tape to hold the assembly together.

Attach leads for the batteries (B1 and B2)-the leads are more easily attached to the actual foil. runs on the foil side of board. Solder 11 inch lengths of wire to the appropriate pads on the printed circuit board for connection to the touch terminals on the handle. Check for accuracy, quality of solder joints, potential shorts, etc. Once you are satisfied that the printed circuit portion of the project contains no construction defects, put the assembly to the side and begin preparing the display portion of the circuit.

Mechanical Assembly. The display portion of the Plasma Saber is comprised of the neon display tube and a clear plastic shroud, with a few additional items thrown in for good measure. Although the following instructions assume you are building the 26-inch version, all procedures described herein also relate to the other Saber display sizes. In any event, begin construction by cutting a 29 1/2-inch length of 1-inch clear plastic tube, which will serve as a shroud (see Fig. 5), and then flush up and debur the ends.

After that, fabricate four spacer rings from a sheet of clear flexible 1/8-inch vinyl as shown. The spacers were manufactured by cutting 7/8-inch diameter circles, punching 3/8-inch holes in their centers, and then cutting them to a triangular shape, as shown in Fig. 5. The center holes of the spacers should fit snugly onto the neon tube, while the outer diameter of the spacers should provide reasonable friction to the inner walls of the shroud tube. The spacers-which should be mounted to the NE-26 display tube as shown in Fig. 5-help to center and hold the neon tube inside the plastic shroud, while simultaneously offering some small degree of shock protection in case the unit is mishandled. Note that other materials can be used to manufacture the shroud.

Next, fabricate three adapter rings, as shown in Fig. 5, from 1/16-inch thick Lexan by cutting three 1 1/2-inch circles and punching a 1-inch hole in the center of each circular piece of Lexan. The outside diameter of the spacers must fit snugly into the handle of the Saber. The adapter rings should be positioned and glued to the shroud tube as shown in Fig. 5. Prepare a plastic end cap by drilling a centered 1- inch hole in it, and then put it in position on the shroud assembly.

Follow that by fabricating a handle from a 10-inch length of 1 5/8-inch (diameter) x 1/16-inch (wall thick- ness) rigid PVC tubing or equivalent material. Start this operation by drilling two small 1/16-inch holes in the handle (as shown in Fig. 6), through which the contact terminals that will mount directly to the handle are connected to the printed circuit board. Note: The positioning of the contact terminals isn't critical and should be placed to suit user preference.

Final Assembly. Insert the NE-26 tube assembly with spacer rings and assembled printed-circuit board into the shroud as shown in Fig 7, Note: It may help to moisten the inner walls of the shroud by deeply exhaling into one end and quickly inserting the neon tube assembly to form the "blade" portion of the Saber,
PARTS LIST FOR THE PLASMA SABER
 

SEMICONDUCTORS
Q I-PN2907 general-purpose, silicon PNP transistor
Q2-PN2222 general-purpose, silicon NPN transistor
Q3~D40D5 NPN silicon power transistor

RESISTORS
(All resistors are 1/4-watt, 5% units.)
R1 - 5.6-megohm
R2 - 1000-ohm
R3 - 4700-ohm
R4 - 680-ohm

CAPACITORS
C1 - 0.1-uF, ceramic-disc
C2 - 10-uF, 25-WVDC, radial-lead, aluminum electrolytic

ADDITIONAL PART5 AND MATERIALS
T1 - High-frequency oscillator transformer (see text)
NE1-NE-26 glass neon tube (see Table 1)

B1, B2 - 9-volt transistor-radio battery
B3, B4 - 1.5-volt AA alkaline battery (optional, see text)
Printed-circuit materials, battery connectors, adapter rings (1 1/2-inch OD x 1-inch ID x 1/8-inch thick Lexan washers, see text), shroud (29 1/2-inch length of l-inch OD x 7/8-inch ID Acry1ic/Plexiglas tubing), spacers (4 7/8-inch OD x 3/8-inch ID x 1/8-inch Flexi Clear PVC, see text), #22 buss wire, #24 stranded wire, 4-inch tie wraps, l-inch clear plastic cap, 15/8-inch plastic cap (see text), adhesive-backed metallic tape, 10 -inch x 1 5/8-inch black plastic handle (see text), wire solder, hardware, etc.
Note: A complete kit of parts is available from Information Unlimited, contact them by snail mail at P.O. Box 716, Amherst, NH 03031; Tel. 603-673- 6493; e-mail: wako2@xtdl.com; Web: www.amazingl.com

Insert the blade portion of the Saber into the handle with the forward adapter ring (the one closest to blade end) recessed approximately -inch into the handle, as shown in Fig 6, Liberally apply hot-melt adhesive or other suitable glue to secure the assembly in place. Allow the glue to cure and then slide the prepared end cap into place as shown, Summoning all the patience and ingenuity that you can muster, thread the touch-contact wires through the two small holes that were previously drilled in the handle, as illustrated. Then sandwich the stripped ends of the wires to the handle using small pieces of metallic tape as shown. Cut pads to shape for appearance using an X-acto knife.

Connect a pair of fresh standard 9-volt alkaline or lithium batteries into the battery snaps and insert them into the handle. After that, pack pieces of foam rubber into the handle to hold the batteries in place. Place a final end cap on the open end of the handle. Test unit by bridging the touch terminals and verify correct operation. Finish assembly by attaching caps into place and attach any labels or decals.

Now go forth and have some fun. The Plasma Saber looks amazing cutting through the air at night or in a darkened room. Just make sure not to smack it into anything with too much force. A shattered neon tube is no fun, and dangerous to boot. However, with some care, the Saber can prove a wonderful source of enjoyment for a long time in this galaxy.
TABLE 1 - PRICE LIST FOR SINGLE AND MULTIPLE PLASMA TUBE PURCHASES
Part List 10-24 25-99 100-249 250-499