Helping a friend put together an ignition system for a potato (or tennis ball) cannon.

1. (+)(-)
Ground
High Tension Lead
Condenser
Coil
Momentary
Switch
Safety Switch

2. High tension lead
Called an “ignition coil wire”
Different than spark plug wire,
It has two male ends
If you don’t want to lug
a car battery around,
you can also use a
jump starter unit
This is how a coil works:
The primary winding builds a strong
magnetic field when energized,
and then transfers the energy to
the secondary winding when the circuit
is interrupted. In doing so, it steps up
the voltage from 12V to 15kV or more.
The charge/discharge cycle can happen
in fractions of a second. A traditional V-8 engine
uses a single coil to do this 20 times per second
when operating at 4800 rpm.
The momentary switch is only “on”
when pushed down. No clicker
switches. Pushing it down energizes
the coil, releasing it creates the spark.
The role of the condenser,
which is really a capacitor,
is to limit current backflow
into the battery. Important.

3. A solid grounding is
important and needs
to be isolated. Use a
wire nut and wrap the
nut/wire with electrical
tape to prevent problems.
The safety switch lets
you turn the system off
so you don’t have an
accidental spark (shock!).
A cheap blade switch mounted
on a piece of wood is good.
Projectile
Combustion Chamber
Cylinder Head
Sparkplug
Rifle Barrel
• Use schedule 20 PVC for rifle
barrel
• Attach reducer/coupler to a
short section of threaded
steel pipe. The projectile will
rest on opening to
reducer/coupler and seal the
combustion chamber
• The cylinder head is a screw-
on steel cap.
• Drill a hole for the spark plug
in the center of cylinder head.
Ask a machinist friend to drill
and tap the threads to match
the spark plug, if possible.
• Drill a second small hole in
the cylinder head suitable for
a screw that will be used to
attach the ground wire.
• Drill a small hole (1/8”) in the
PVC reducer/coupler above
the combustion chamber.
This will be used to fuel the
chamber.
• You can use any light aromatic
fuel such as
gasoline, butane, methanol, et