Step 1: Parts You Will Need
First of course you will need an ignition coil. These are easy to find new or used. The common metal can type can be had new from J. C. Whitney http://www.jcwhitney.com/autoparts/Search?catalogId=10101&storeId=10101&sku=ignition+coil for $18 and up, or you can buy them used from a local salvage yard. I got mine off eBay for about $9. They are Accel "Super Coils," capable of up to 50,000 volts. I recommend you get high value coils, if only for the better quality and higher safety factor.
Next you will need a household dimmer switch. Get the simplest, cheapest one you can find. Mine cost about $8 from Home Depot. It is rated for 600 watts. Get a cheap nylon cover plate to match it.
You will need a film capacitor. More about this critical component later.
You will need some appliance cords. I salvaged some from a toaster oven and a box fan. Leave the wall plug on one cord, as this device runs off household 110 volt AC current.
You will need a box of some kind to mount your components in. I had a wooden craft box lying around (bought at Michaels craft store for less than $5). It measures 8.5" by 5.5" by 3.25" and has a hinged lid. This proved to be a convenient size. You can get a electronics hobby box from Radio Shack if you like, but the wooden boxes are cheaper.
Other bits and pieces you will need:
(1) half inch PVC pipe end cap
about five inches of smooth metal rod, 5mm or 3/16 inches in diameter. Copper or brass is best.
(1) 10-32 size brass thumb nut
(1) 10-32 threaded aluminum standoff, one inch long
(1) 10-32 threaded rod, about 3 inches long
(2) brass cotter pins, 2.5" long
(2) alligator clips, preferably insulated. Mine in the photos aren't, so I had to sheath them in vinyl tubing.
(2) crimp-on ring connectors, sized to fit the cords you are using
(2) crimp butt connectors, ditto
a length of half-inch square basswood (four inches will do)
a spring clothespin
a wood screw that will fit through the pin's center spring
a couple of sturdy rubber bands
(2) small nylon zip ties
electrical tape, a crimp tool, wire stripper, etc.
Step 2: The Switch Box
Measure the underside of your dimmer switch and mark out a rectangle on the lid of the box. Bore holes at the corners of the rectangle, then with a keyhole saw or X-Acto saw cut out the rest of the rectangle. Mount the switch in the lid with two short wood screws. Install the trim plate over the outside of the switch.
With the lid open, take the length of half inch square basswood and glue it across the lower right corner. Allow glue to dry. Take the clothespin and place it on its side about halfway along the basswood brace. Screw an inch long wood screw through the center "eye" of the hinge spring. Don't tighten too much. Let the clothespin pivot a little.
Bore a hole low but centered on one end of the box. Feed in the electrical cord you have with the wall plug attached. Divide the cord about four inches or so (that is, using a sharp knife divide the molded plastic cord into two strands). If the cord is marked, or if you can trace the positive wire, attach it to one of the dimmer switch wires with a crimp-on butt connector. The other wire is the negative line.
Take about eighteen inches of two strand appliance cord. Remove any plug and strip all four ends. Split one end of the cord about eight inches. Bore two holes of appropriate size in the opposite end of the box and feed in the two strands separately, one in each hole. Connect the lower wire--the one by the wall of the box nearest you--to the power cord ground line using a crimp-on butt connector.
Split the other end of the output leads about three inches and crimp on two ring connectors sized to fit the coil's 10-32 terminals.
Attach an alligator clip to the as yet unused wire coming from the dimmer. Attach the other alligator clip to the other output wire. Put a nylon zip tie on the output leads inside the box, and a similar tie on the wires outside, to relieve any pulling stress on the wire. Cut a thick rubber band in two. Tie this several times around the power cord where it enters the box, also to relieve stress on the cord.
Step 3: Prepare the Coil
Bore a 3/16ths hole in the center of the PVC cap. This should allow your 3/16 rod to slide in, but be close-fitting.
Insert the brass rod in the PVC cap and press the cap over the coil's high voltage center post. Make sure the brass rod is in good contact with the bottom contact.
Screw the threaded rod into the aluminum spacer. Fit a brass cotter pin to the top of the brass rod and the threaded rod.
Slip the positive output lead over the positive (+) marked terminal on the coil. Use a brass thumbnut to secure it.
Put the negative output lead on the (-) marked terminal. Screw the aluminum spacer--threaded rod assembly over it and snug it down.
Step 4: The Capacitor--updated
Use a film capacitor rated for at least 100 volts and 1uF. 100v may seem low, but the cap I used with greatest success is rated at 100v, while higher voltage rated caps only made the coil buzz.
DO NOT USE ELECTROLYTIC CAPACITORS! These are the ones that look like little metal cans. Even high voltage, high capacitance types failed on me *every* time. They got very hot, they bulged, and one even exploded. No joke--leave them alone! Stick with film types. There may be others that will work as well, but my best result came with a cap salvaged from a computer monitor circuit board.
It is marked:
I measured the capacitance of this cap with a multimeter and got a reading of 2.178 microfarads. With this cap I get good long sparks, cool operation, and no overheating of the coil, the switch, or the cap, although I have not run the coil for long periods. The longest I have let it run was for 60 seconds.
I mount the cap in the jaws of the clothespin. The lower lead goes to the output cord alligator clip, and the upper lead to the dimmer switch.
Update as of 3/23/09: I obtained some metalized polyester capacitors rated at 2.2 uF at 250V from an eBay seller . They work very well, and give a greater range of output on the dimmer (by this I mean I get more sparks at higher settings than I did with the 100v cap). The new caps are marked:
Tolerance is rated at 10%.
Update as of 4/22/09 While tearing down a surplus microwave oven, I came across an array of capacitors I thought I would try in the coil driver. (These are not the usual metal can capacitors associated with microwaves; they are high voltage film-type caps). A couple of 3KV caps did not work, but a 4.5 uF, 250 volt cap gives much greater sparks than anything I have used before. Discharges are longer, thicker, and louder. They are no longer blue-white, as with the previous caps, but a kind of vivid orange. Does this indicate a lower temperature, I wonder?
The cap is marked:
Update as of 4/18/10: Even better results have been obtained using a 10 uF, 330 volt VIOT motor start-run capacitor. This is a large, oval metal-bodied cap I found on eBay. With the motor start capacitor the coil produces much more powerful arcs. I have used this coil and cap combination to power my Tabletop Tesla Coil with success. If you can get a motor start cap like this, by all means use it.
Step 5: Firing It Up--and What You Can Do With It
With the switch off, plug the power cord into a wall socket. I chose a cord that happens to have a RF (radio frequency) choke built into it. This helps damp any high power surges back up the wires into the house circuit. If your cord doesn't have one, you can buy clamp-on chokes at Radio Shack. These attach over your existing cord.
The dimmer switch has surge protection built into it. I understand this is normal for all modern dimmer switches. Read the packaging in the store before you buy. Some dimmers are rated for 600 watts, other for 750, etc. 600 seems to be adequate in my setup, even with the Accel Super Coil.
I also take the precaution of plugging into a GFI (Ground Fault Interrupt) wall socket. If there is a short, the breaker will trip.
OK, you are plugged in. Keeping a healthy distance from the electrodes, bump the dimmer switch on. I get the brightest, loudest sparks at the lowest setting of the switch. Pushing the slider up attenuates the spark until it eventually disappears, leaving only a strong humming. The coil is still active at this level, it's just not producing enough power to jump the air gap between the electrodes.
The gap affects the appearance and noise of the spark too. A close gap--say, an inch--gets you a loud, continuous blue-white spark. Opening the gap (CURRENT OFF, PLEASE!) thins the spark.
ALWAYS TURN OFF THE DIMMER AND UNPLUG THE POWER CORD BEFORE MAKING ANY ADJUSTMENTS TO THE COIL! This thing puts out, at my best guess, around 30,000 volts. It runs off household AC. This is a deadly combination, so don't screw around with it, OK?
THE CAPACITOR RETAINS A STRONG CHARGE EVEN AFTER THE DEVICE IS SWITCHED OFF AND UNPLUGGED. You can add components to prevent this, but my goal was simplicity. You can safely discharge the capacitor by touching both leads at the same time with the shaft of a WELL INSULATED screwdriver. Holding the tool by the insulated handle, short-circuit the leads. You'll see a flash and maybe hear a slight pop. Once the cap is discharged you can remove it, swap it for another, etc.
What can you do with the coil?
Sparks are fascinating, but there's more to be done. I have used the coil to power a "lightning ball," consisting of a clear incandescent bulb mounted on a porcelain base. If you attach one lead to the screw terminal base and the other to an outside terminal made from a bend of heavy wire (like a length of coat hangar) you can make a plasma ball. Unlike the cute plasma spheres you can touch, do not touch this one!
Someone's bound to ask if you can power a Tesla coil with this outfit. You will need other components to do so, but I would say yes. I haven't tried it, but I have read about others using ignition coils as power supplies for Tesla coils.
A search of the net can turn up other uses for a high voltage power source.
BE SAFE, PLEASE. Use good sense and good technique when handling high voltage equipment.
Update 12/10/2013: Here's another use for the spark coil-dimmer switch apparatus:
Step 6: Addendum: a Simpler, More Compact Version of the Coil Driver
And it works just fine. The rotary switch is pushed in to be "on," then adjusted by twisting the dial. It seems to have a greater range of operation than the sliding switch.