Looking for Solutions
The modern “single coil per cylinder” scheme used in newer cars uses, as the name implies, a separate coil for each cylinder that is activated by a small 5-volt logic signal from the engine computer. Starting with one of those coils would have several advantages: built in power transistor, built in trigger mechanism, and small size and light weight.
Using just three resistors, a General Motors LS2 engine coil and a capacitor, I created a pretty nasty buzz coil. By nasty I mean a much longer arc and a much faster spark rate than the old Model T-type buzz coil. After building one for Jim and another for myself, I put the project on the shelf. In fact, never expecting to return to it I threw the schematic away. Recently, however, the complex 555 timer system came up again on an engine website. That inspired me to find out how Jim’s was doing, and it turns out he has been using it for four years without a problem. Together with Evac 1 being so inexpensive and simple, I thought it was time to get it back off the shelf and try to remember the resistor and capacitor values. One key point is that it uses no pesky expensive semiconductors (transistors, integrated circuits, diodes, etc.) that can be put in backwards and blow up.
Make the Coil
Figure 1 shows one of the coils I picked up at a local junk yard. Four coils, with the wire harness, (one bank of a V8 engine) was very cheap, just $20 at the time. You can also find new coils on the internet for $20 or less. An advantage of the junk yard route, even if you only need one coil, is that getting the wire harness gives you the coil connector plugs (pigtail).
For those who might be interested, Figure 2 is a simplified schematic of what’s inside a GM LS2, part no. H6T55171ZC coil. At the junk yard it would be common on many 1999 to 2006 Chevy and GMC V8 engines. I found them on a 1999 Chevy Silverado Z71.
Figure 3 is the very simple Evac 1 schematic. The two 15k? (ohm) resistors can’t be put in backwards nor can the 4.7µF capacitor if it’s a 50-volt version. If your capacitor is higher voltage, say 200-volt capability, it’s likely polarized and will need to go in oriented as shown. The 2.2? resistor needs to be at least 5 watt, but the two 15K? resistors can be any wattage.
The components I used are shown in the lead image. If your coil doesn’t have a connector plug you can cut some plastic back and get to the pins. If you orient the coil as I have, the pins left to right will be red, green, brown, black. The blue tube is a 14-16 gauge butt connector, it slides on to the coil’s high voltage pin very nicely.
Although the four components can be soldered and left hanging on the coil wires, I like to use a small prototype or vector board to make things more ridged and easier to secure in the box. Figure 4 is the assembly with the plug, ready to go in the box.
As shown in Figure 5, everything is in the box and ready for the lid to be glued on. As a last step, I have been squirting a bit of that infinitely expanding insulating foam into the box. Doing so keeps everything in place as it gets bounced around.
Results
Once completed, you will notice some differences between a Model T buzz coil and Evac 1. Generally, Evac 1 will throw a spark two times longer than a buzz coil, sometimes up to 3/8 of an inch. Both take about 2ms from power on till the first spark. After the first spark, however, the buzz coil typically throws 500 sparks per second while Evac 1 throws 4,000 per second. The buzz coil buzzes, while Evac 1 howls. Also, a buzz coil generally doesn’t care which battery terminal is positive, but Evac 1 does so I have colored the thumb nuts (red for positive).
The above information should give you a very nice buzz coil. I have found that the capacitor is only needed in very unusual operating conditions, but as there are conditions where it is needed I have included it in the schematic. If you feel your operating conditions are near normal, leave it out.
Most observable parameters stay about the same, except time to first spark drops to 0.5ms, way faster than a Model T buzz coil. When the capacitor is left out, Evac 1 becomes even simpler: three resistors and a coil. Also, almost any LS2, 3 or 4 engine coil will likely work. Figure 6 is a capacitor-less unit built on a LS3 coil, part no. 12573190. For this build I used a Dremel tool to cut the hood and expose the pins. The three resistors and the ground wire were directly soldered to the coil pins. It would be a good idea to use shrink tubing or spaghetti tubing on the exposed leads.
The Evac 1 schematic shown is for intermittent use only, as on vintage gas engines. If used continuously for say one minute continuous “on” time or more than 30 percent duty cycle, the 2.2? resistor power rating should be raised to 15 watts.
The 4,000 or so sparks per second allow very little dwell time for the coil, thus limiting the output voltage and arc length to around 3/8 inch. If you desire a show and tell only coil, increase the capacitor to 6800µF (or some large value). The rep rate will drop to 50 per second, but arc length can exceed 3/4 inch. It will, however, not be useful on an engine as the delay to first spark becomes large, thus retarding the engine spark to the point the engine won’t run.
Sometimes connection to a spark plug engine can be confusing. Figure 7 is the connection used by John Deere on their spark plug engines (12-volt rather than 6-volt, Evac 1 rather than buzz coil). That is the John Deere recommendation. The order of the battery, kill switch and the engine ignition switch are however arbitrary; you may interchange their location at will.
Engine enthusiast Dave Cave is a retired electrical engineer living in Arizona. Contact him at JDengines@cox.net
