(You can build them yourself from kits for peanuts)
"Most carburetor problems are electrical"
That was passed on to me by a savvy old auto mechanic back
in 1956 and it has proven true more times than I can remember.
The standard Kettering points/condenser ignition timing setup
works just fine if the point faces are parallel and clean, are
closed with proper pressure, and the condenser (capacitor is the
current term) is good and of the correct value. A lot of ifs,
don't you think? Also, unfortunately for us model engine builders,
either a grossly oversize points/condenser set from older style
lawn mower engines must be used or a miniature points set will
have to be fabricated from questionable materials and with questionable
accuracy. Most model engines don't have shaft oil seals and just
a little oil leakage onto the points will cause major problems.
Ever wonder why so many model gas engines on display at shows
are never ran? Do you suppose it's because they are easy starters
and good runners? Some may be. But how many really otherwise great
engines won't run or are so hard to start because of ignition
problems, that the owner won't even bother? What a shame! If you
don't mind having your engines ending up as just shelf models,
then read no further!
I came across the answer some years ago in a magazine article
written by Floyd Carter and all my spark plug ignition model gas
engines use it with great results. The original Transistor Ignition
Module (TIM - 4) is a simple two transistor circuit that can easily
be home built. TIM-4 was designed to operate on 3.6 volts (three
Ni-Cad cells in series). It eliminates most, if not all the problems
of standard points systems. The coil will give a good hot spark
every time. The circuit requires very little current to trigger
(25 ma). This allows use of a tiny micro switch for the points
which can be easily hidden. There is no arcing, so the contacts
in the micro switch will never burn. If you want your engine to
be authentic, or on already built engines which you don't want
to change, the old fashioned point set can be used if desired.
A "condenser" is not needed but can be included for
looks.
And now for the really BIG advantage........
Since we now have a circuit that is so easy to trigger, we can
use a tiny magnetic sensor instead of (heavy duty switch) mechanical
point contacts! The magnetic sensor is called a "Hall Effect
Device". They are really tiny, measuring just .125"
x .165" x .060" thick (3mm x 4.2mm x 1.52mm). Instead
of a cam to operate contacts, a tiny magnet (only 1/8" diameter
by 1/16" thick - or smaller) mounted on a drum or disk (cam
gear) triggers the Hall device which is mounted in close proximity.
The Hall sensor is located remote from the circuit board which
can be hidden under the engine, or wherever you wish. Now you
have the ultimate in small and reliable ignition, no mechanical
parts, rub blocks or contact points at all! The circuits are extremely
reliable.
Floyd designed different systems for different voltages and engine
types. He is a retired aerospace electronics expert now enjoying
life and intends to continue doing so. He sells TIM-4 units ready
made. He does not make any of the units available as kits, nor
the Electronic Buzzcoil ready made. There is actually nothing
at all difficult in building these circuits except a little care
and the exercise of some common sense. With some help and advice
from Floyd (and against some!) I am making these kits available
under the following conditions: If you are not profiecent at soldering,
don't have a 25 to 35 watt (max.) soldering pencil (no 150 - 300
watt solder guns), don't have some previous experience with electronic
parts and circuit boards, then you probably shouldn't order these
kits because I positively will not replace
any damaged part at my expense for any reason. I will sell replacements
for damaged parts at very reasonable prices in the unlikely event
that you should need them.
I changed some of the components of the original TIM unit
for 6 volt operation on stationary engines. I designate this as
the TIM-6.
To run an engine with electronic ignition you will need:
TIM-6 module, a suitable 6 volt ignition coil (Exciter), spark
plug and a good 6 volt battery that can supply at least 4 amps.
Important Note
- These ignition modules may be used
on multi cylinder engines if model coils such as the Exciter,
Modelectric or Gettig and having a primary winding resistance
of Approx. 1 ohm. This combination runs my V-Four engine at 6,500+
RPM with no problem at all.
Consider use of other type coils to be experimentation on your
part. I advise against the use of automotive ignition coils without
an in line ballast resistor with these electronic ignition modules
because most of them - maybe all of them - draw more current than
these circuits can handle. If you do, use a ballast resistor so
the current draw is not over 4.5 amps.
TIM-6c
- The new and slightly larger board
dimensions has more distance between solder pads for easier assembly.
A reverse polarity protection diode has also been added to the
circuit.
For use with 6 volt batteries (or 4.8 minimum to 7 volts
maximum) on any engine type. The kit contains: (1) TIP42C Transistor,
(1) 2N2907A Transistor, (4) Resistors, (1) LED Timing Light, (1)
3 Amp. 40 volt diode, (1) Hall Effect Magnetic Sensor, (1) Length
of Heat Shrink Tubing to insulate the Hall Sensor leads, (1) Rare
Earth Magnet, (1) Drilled Printed Circuit Board, Circuit Diagrams
and Construction Notes. The circuit board is approximately 1.35"
wide by 1.73" long. Just to the left of the circuit board
is the Hall Effect sensor and the dot just above it is the rare
earth magnet which is just 1/8" in diameter and 1/16"
thick (3.2mm x 1.6mm).
New Deluxe TIM-6c - Same as above,
except also
includes a high quality PCB Screw Type Terminal Strip for ease
of connecting or removing your battery, coil, and other wires.
Ignition Dwell Time
- A rule of thumb to calculate dwell angle is Cam
Shaft RPM x .0075 for 4 cycle engines, or Crankshaft
RPM x .0075 for 2 cycle engines. This will determine the
shaft rotation in degrees that the coil should be energized (points
closed or Hall Sensor turned "ON"). A little experimentation
with your Hall Sensor and magnet (if different than below), plus
a simple calculation or two will determine the radius from the
center of the shaft to mount the magnet and Hall Sensor. High
speed engines need small radius (or several magnets in an arc)
to get enough dwell angle, slow running engines require greater
radius (or a smaller magnet) to prevent excessive dwell angle.
Set the dwell angle for the highest expected engine speed. Therefore,
if a 2-cycle engine has a top RPM of say 6,000 RPM, then .0075
times 6,000 = 45 degree dwell angle. In the above case, draw a
45 degree angle with that circle drawn centered at the angle convergence
point. The 45 degree lines at that circle is the length of arc
you need to have magnets. For a 4-cycle engine at 6,000 RPM, use
the cam shaft speed of 3,000 RPM which gives a 22.5 degree dwell
angle. Too much dwell angle causes extra drain on the battery
and worse, heats the coil. Too little dwell angle will cause the
engine to missfire at high RPM. Magnet diameter needs to be considered
as well. Try to get it right. See the diagram below.
TIM-6 magnets are 1/8" dia. by 1/16" thick. With the
1/8" dia. rare earth magnets at .030" away from the
Hall Sensor face, the sensor will be turned "ON" during
the time it takes for the magnet to move .200" across the
sensor face. In other words at a certain point, as one edge of
the magnet starts to move cross the Hall sensor face, the sensor
will turn "ON" and stay "ON" until the 1/8"
diameter magnet has moved across the Hall Sensor face for a distance
of .200". As the magnet moves beyond that point, the Hall
sensor will turn "OFF" again. The distance the magnets
move during turn "ON" does not change significantly
with the magnets from .025" to .035" from the Hall Sensor,
so distance away is not that critical.
For the 2mm diameter magnets I formerly had available, at a distance
of .030" away from the Hall Sensor face, the Hall Sensor
will be turned "ON" during the time it takes for the
2mm dia. rare earth magnet to move a distance of .120" across
the face of the sensor. All these measurements were made using
the DRO on my milling machine so they are very accurate.
For 2-cycle model aircraft engines, your particular engine
prop driver/crankcase nose configuration will determine the minimum
circle diameter you can mount a Hall Sensor and magnet or arc
of magnets to achieve the needed dwell angle. If the Hall sensor
can be mounted so that it can be rotated around the center of
the shaft, ignition timing can be adjusted for "advance"
or "retard".
More info is in Strictly I.C. magazine #27 and #36. Back issues
available robert@strictlyic.com
PLEASE NOTE - I get asked a lot of
questions about using these ignition modules on chain saw, weed
eater and other non-stationary and/or non-model engines. Many
of these types of engines have been converted and are operating
in various applications. However, I make no claim of suitability
of any of the above ignition units for non-model engines. Some
of these engine types may be suitable and some may not be. If
you want to convert these engines, you are on your own, so you
should consider the use of these ignition modules and/or coils
on non-model engines to be experimentation on your part. Please
also note - electrical items are not returnable for refund for
obvious reasons.
With the above understanding, you want to go ahead anyway,
here are some guidelines. With the right battery voltage and -
very important - an ignition coil with a primary resistance of
not less than 1 ohm, I see no reason why an enterprising person
shouldn't be able to convert most, if not all, of these engines.
In a nutshell, determine from your engine the minimum circle diameter
you can use to get the proper dwell angle (see above) from the
rotating magnet(s) which are mounted on a drum or disk somewhere
on the crankshaft (2 cycle) or cam shaft (4-cycle) and suitable
mounting of the stationary Hall Sensor in close proximity to the
rotating magnet(s) and make that installation. That is all the
modification you need to do to a single cylinder engine. There
are many ways to set up multi-cylinder engine ignition systems.
Usually multiple magnets and a distributor are required. Again,
I don't do consulting so you are on your own. Also, if you intend
to use the engine with radio control, remember that the entire
ignition system - TIM module, coil, plug wire, plug, etc. should
be shielded and grounded to the engine to prevent radio interference
and possible loss of control of your model. On the other hand,
I have talked to some fellows who say they have not found this
necessary with their particular radio by installing the radio
as far aft in the plane fuselage as possible.
You decide what you are comfortable with. Good
luck....
