Introduction: Simple Method of Getting a Spark Plug to Spark


I was working on a Halloween prop that was going to have a flame shoot out of it periodically so I needed some type of igniter to light the flammable liquid that would produce the flame.  My first thought was to use a spark plug to do the job.  I have a lot of old car parts lying around so I could assemble something from that.  Bulky, yes for sure, but basically free for me to implement.  Since the prop is going to be static, it didn’t matter how big my solution to this problem was going to be so I went ahead with it.
Well I never finished the prop but I ended up with the ignition part of it which I’m sharing in this Instructable.
Basically I’m building a very simple single cylinder ignition system.
There are much better explanations of how automotive ignition systems work but here is a brief explanation of what you are going to see in this instructable.

How an ignition system works

When the points are closed, current flows through the battery, across the closed contact points through the primary circuit in an ignition coil. 
When the points open – this breaks the circurt between the battery and the primary windings, the magnetic field that's in the ignition secondary coil collapses and induces a high voltage into the secondary circuit of the ignition system.  The voltage is high enough (15k to 24k + volts depending on the coil & conditions) to cause electricity to arch across the spark plug.   

The condenser (basically a capacatior) is in place because when the magnetic field collapses in the primary circuit, a short voltage spike is introduced to that circuit as well. The condensor/capacitor absorbs this voltage spike and prevents an arch of electricity across the open points.  (Which over time will damage the contacts on the points preventing them from completing a circurt when the points are closed?  )

The amount of voltage that it takes to shoot an arch of electricity across the electrodes in a spark plug is in the TENs of THOUSANDS of volts.  If you work on cars/motorcycles/lawn mowers, etc.  You’ve probably been zapped by a spark plug.  It really sucks, no other way to put it.  Since the spark plug is exposed in this project – you need to be CAREFUL with what you are doing.

1. 12v automotive coil
2. One set of automotive breaker points
3. One automotive condenser
(In my example all 3 of these are from a 1960s Volkswagen) you can probably mix and match from what you have and it won’t matter.
4. Spark plug – any will work.  I actually bought a small gas engine spark plug for the project I was working so it would fit in the prop, but any spark plug should work.
5. Something to bolt the thing together on – a 2x4 in my case.  Some wood screws and tie wraps, wire and connectors to hook it up.
6. A spark plug wire – you could use a regular wire for this but getting it to stay in the coil’s output and keep a connection and stay connected to the spark plug can be a bit of a pain.  Chances are if you have the other parts laying around you will have a old plug wire. 
7. A way to open and close the points.  For this I used a small Tamiya motor and gear box i had laying around in my robotics parts box.  The triangle bit is the hub from a Tamiya rubber wheel. Makes for a nice 3 lobe came to open and close the points.
8. A battery.  For this instructable's demonstration I’m using a 7.2 v RC car battery which barely produces any spark.  For best results, a 12v DC battery would be the way to go.
So those of you in the know you might be asking – what about a ballast resistor for the coil?  If your coil was using a ballast resistor go ahead and use it.  I know mid-60s Fords used them, my VW coil in this example didn’t.
What about resistor plug wires, resistor spark plugs?   Well none of that really matters to get a decent spark out of the spark plug for example purposes.

The use of the 2x4 came from a way to get the gear box close to the points to open and close automatically and also include the coil.  The Taymiya gear box’s output fit nicely by being mounted at the end of the 2x4.  I mounted the points so that I could swing them closer to or farther away from the hub on the gear box to make sure they will open.  The points don’t need to open very far (the spark gap – how far the points open on a VW bug is like .028 thousands of an inch).  Since there was flex in my gear box assembly I made the gap a lot bigger.

As you can see from the picture, everything is connected to a sheet metal plate attached to the 2x4.  The sheet metal plate gets used for a common ‘—‘ minus.
Battery:  ‘+’ goes to the ‘+’ on the coil.  ‘– ‘goes to the common ‘–‘
Condenser:  The condenser connects between one side of the points and ‘-‘.  The body is bolted to the common ‘–‘ and the lead of the condenser is connected to the lead coming from the points which goes to the ‘-‘ of the coil.
Coil:  ‘+’ comes from the ‘+’ on the battery, the ‘–‘ connects to the points and the condenser lead.  The output of the coil (the spark plug wire connection), connects to the spark plug.
Points:  The points are basically a mechanical switch.  The wire runs from the ‘–‘ on the coil and the body of the points connects to the common ‘-‘. 
Spark plug:  The output of the coil goes to the top of the spark plug (via the spark plug wire).  The body/threads and ground electrode go to the common ‘-‘. 
Gear motor:  One end of the motor connects to the common ‘-‘, the other side of the motor connects to the ‘+’ of the battery (in this example, the ‘+’ side of the coil).
If you go the gear motor route and 12v, I don’t think the Taymiya motor can handle 12 volts.  You may need a separate 3-6 voltage source to drive the motor.  It really screams at 7.2 volts and I don’t know how long it will last.  Initially I was using two D cells to drive the motor, which worked but didn’t look good from an example point of view so i ditched it for the pictures.
Circuit walk through
For the coil, the primary winding is the ‘+’ and ‘–‘ in the coil.  The secondary windings are the coil’s output (spark plug wire) and the ‘–‘ on the coil.

Looking at my picture you can see that for the primary of the coil, the ‘+’ from the battery goes to the ‘+’ of the coil, through the primary windings, out the ‘–‘ of the coil to one side of the points. The points contacts touch (closed switch) and the other side of the points (the body of the points) go to the common ‘–‘.  When the battery is connected current is flowing through the primary coil.  When the points are opened up, breaking the circuit in the primary, this is when the secondary’s magnetic field collapses, voltage increases to such a point that it’s high enough to shoot a spark across the air gap in the spark plug between the center electrode and the ground electrode.  If the spark plug was in an engine, with the compressed air/fuel mixture, this spark would ignite the gas, causing it to expand, pushing the piston down. The condenser prevents a small arch across the points (the primary winding of the coil) when the circuit is opened.
When the gear motor runs, the hub on the axle of the gear motor pushes the points open, causing the primary to break and the spark to spark.
Quick video of the system working.