A
Cheap Electronic Ignition System
for
Low Speed Engines.

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This article describes a cheap way to have electronic ignition for your old engine.  It uses commonly available components and an ordinary 6 Volt or 12 Volt points-type auto ignition coil.

The system works with a small 12 volt gel-cell or lantern battery and will power the ignition for many hours before replacement or recharging of the battery.

Schematic of "Fire on Close" version.

Briefly, here is how it works.  When the points close, a negative-going pulse is coupled to the base of Q1, causing it to turn off for a millisecond or so.  This pulse is fed to the gate of Q2, an International Rectifier Hexfet transistor or equivalvent.  Q2 pulls the primary of the ignition coil down for the one millisecond or so, which is plenty of time for the core of the coil to saturate.  When Q2 turns off, a resonant damped oscillation is created between the primary of the coil and C3 and this causes the spark.

Notes:
  1) Pulse length at the collector of Q1 is determined by C2, R2 and R3.  Raising R3 lengthens the pulse.  
  2)
R2 also acts to limit the base current of Q1.
  3) D1 provides the discharge path for C2.
  4) D2 and D3 may not be needed but are there to shunt positive and negative pulses coming from the gate of Q2.
  5) C4 may not be necessary but is used to capture any transients that may be present on the 12 Volt line.
  6) A heat sink is not necessary for Q2.  In normal use, since the duty cycle is so short, it doesn't generate any noticeable heat.
  7) Q2 should have a peak voltage rating over 400 Volts.
  8) This circuit works equally well with
a digital Hall-Effect sensor, a reed switch or any other device that will pull down or up.  Simply connect the output of the Hall sensor to where the points would connect and use ground and 12 Volts from the circuit board.
  9) Components that have a base drawing must be oriented correctly.  Resistors and all but C4 can be inserted either way.
10) This circuit is not designed for four cycle engines running at over about 2,000 RPM or two cycle engines running over about 1,000 RPM.  Higher speeds will experience some timing lag which may impact performance.

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In response to a numerous request, I've also included a circuit that fires the coil when the points open (like in a conventional point-condenser-coil system).

Schematic of "Fire on Open" version.
Here's how it works.  While the points are closed, Q3 is turned off which means that the collector is at 12 Volts.  When the points open, Q3 turns on, and it's collector goes to ground.  The transition from 12 Volts to ground is coupled through C2 and R2 to the base of Q1and briefly turns Q1 off as C2 discharges through R2 and D1.  This causes the collector of Q1 to rise to 12 Volts for a millisecond or two then, when C2 has discharged, R3 turns Q1 back on, making it's collector return to ground.  The short 12 Volt pulse turns Q2 on and, when the pulse goes away, the coil fires.

Here it is, installed on The Mighty Hoyt-Clagwell.
The entire circuit is enclosed in the little box to the left of the coil in the photo above.  Q2 is bolted to the lid of the box, providing some heat sinking.  Make sure to ground the frame of the coil to the chassis as well as the common ground of the circuit.

As of this date, so far I've only got a couple of hours of run time on the circuit but there's no reason it won't work for a long time.

If you notice any more mistakes or have problems, contact me and I will try to solve them.

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8 July 2014:
Here is a parts list that will cover both versions of the circuit.  I've named DIGIKEY.COM as the supplier but you may use MOUSER.COM, Radio Shack or anyone else to get the parts.  The parts I've got listed are available in quantities of one.  The total amount will be under $5.00 not including shipping, minimums, etc.

Circuit Component Number     -     Description          -        Value        -         DigiKey Catalog Number   -   Price         
                 C1, C2                                        Metal Film Capacitor                 0.1 Mfd @ 50 V                              P4525-ND                          0.29 Each
             
      C3                                           Metal Film Capacitor                 0.22 Mfd @ 630 V                        PCF1578-ND                              0.72 Each
                    C4                                   Aluminum Electrolytic Capacitor         220 Mfd @ 16 V                             P5530-ND                          0.30 Each
                    D1                                               Silicon Diode                             1N4148                                   1N4148FS-ND
                           0.10 Each
                 D2, D3                                   Silicon Diode 1A @ 1,000V                 1N4007                                         641-1312-1-ND                             0.11 Each
              R1, R6, R8                    10K Ohm, 1/4 Watt Carbon Film Resistor      10K Ohm                                    10KQBK-ND                           0.10 Each
              R2, R5, R7                    4.7K Ohm, 1/4 Watt Carbon Film Resistor     4.7K Ohm                                   4.7KQBK-ND                            0.10 Each
                    R3                           68K Ohm, 1/4 Watt Carbon Film Resistor       68K Ohm                                    68KQBK-ND                            0.10 Each
                    R4                           1K Ohm, 1/4 Watt Carbon Film Resistor           1K Ohm                                     1KQBK-ND                              0.10 Each
                Q1, Q3                                     NPN Bipolar Transistor                         2N4401                                 2N4401-APCT-ND                        0.19 Each
                    Q2                            International Rectifier Hexfet Transistor          IRF840                                   IRF840PBF-ND                        1.55 Each
 
NOTE that the part numbers I show are specific to DigiKey. If you order from another company, the DigiKey catalog numbers will not apply.

You can use any means you wish to hang the circuit together.  Just make sure that no unwanted connections are made and that the works are more or less vibration proof.  If you can make PC boards, let me know and, if enough people want it, I will draw-up printed circuit artwork.

Have fun!
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10 January 2016:
This is a continuation of the above page.  I can't imagine that anyone would want to build this but I put it here in case someone decides to try something like it.

Here's another version of the cheap ignition.  This time, I've combined both the fire on make and fire on break versions and have powered it from the AC line.

The AC line power is for those times when you are running an engine where you have power handy.  It saves having to buy and maintain another battery for occasional use.

I started out with the first two circuits and then combined them, using a double pole, double throw switch to make the changeover from fire on make to fire on break.

You will notice in the photos below that I've added a twist.  Since I don't anve any more auto coils and am too cheap to go out and buy one, I rooted through the box of magneto parts from deceased lawnmower engines and came up with a coil that works with this circuit.  Don't ask me what it's out of - I don't have the foggiest idea!  It was originally mounted on an aluminum plate with the points.  I discarded that part, removed the coil from the armature and substituted a straight-line armature out of another deceased magneto coil.

In the circuit, you will see that I've marked R6 with an asterisk.  Changing the value of this resistor changes the "dwell" of the circuit.  The coil I'm using has a primary resistance of around 0.8 Ohms and seems happiest with a dwell of around 1.0 millisecond.  Making R6 higher in resistance lengthens the dwell time and vice-versa.

The power supply consists of a 120V to 12V transformer that is good for a continuous one Amp. drain.  I use a bridge rectifier and large electrolytic capacitor as a filter.  If you do the calculations, the no-load voltage at the filter capacitor is around 17 volts so, when the coil fires, the instantaneous curent the coil is around 21.2 Amps.  This is no problem to the transformer because the filter capacitor provides that amount of current for the one millisecond it is needed and, anyway, the duration is short enough to not stress the transformer.
 
    The unit under test.                                                                    Here is how the guts are laid out.
I've liberally applied hot glue to various components to negate the jiggle factor in moving it around.  There is a safety spark gap at the coil output that is set to around 3/8 of an inch.  This does two things.  It prevents overstress of the insulation inside the coil if the plug wire becomes disconnected and also keeps the flyback pulse (at it's maximum if there is no spark gap to fire across) from killing the output transistor.  I've measured the pulse at over 1,000 volts when there is no safety gap.  Since the transistor gets really unhappy at over about 500 volts, you can see that the smoke will leak out if there is no way to limit the pulse height.

Since I still have some wire wrap supplies left over from when I was in business, I used vector board and wire wrapped it.

The case is made out of 3/8" strip flooring with the backing side out.  The material is basically Masonite with a pretty face.  It can be cut, milled, drilled and tapped (with care) so it makes a nice material for this use.  Of course, we have a few boxes of this stuff left over after "re-paving" our house a few years ago.
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14 January 2016:
After posting a link to this thread on Smokstak, a user with the handle "Odin" came up with a couple of useful changes.  They consist of adding a diode in series between the coil negative lead and the drain of the transistor Q3 and moving the damping capacitor (C4) from directly across the coil to between the coil negative lead and the source of the transistor (circuit ground).

Revised schematic of ignition.
The reason for the change is to relieve electrical stress on the internal blocking diode of the Mosfet.  The damped oscillation of the coil/C4 resonant tank circuit contains a lot of energy, enough to severely stress the internal diode inside Q3 (not shown), causing it's short circuiting in some instances.  Odin's fix for this was to add the diode (D6) to block the negative-going halves of the oscillation, thus removing the stress on the internal diode inside Q3.  The capacitor (C4) was moved to it's present location in order to retain a healthy damped oscillation in the coil.

I then took it upon myself to add two diodes as protection for the trigger input.  These little diodes are cheap insurance against an induced ignition pulse wiping out Q1 or Q2.     

 
Waveform at anode of D6.                                                 Waveform at cathode of D6.
Vertical sensitivity: 50 Volts/division
Horizontal: 0.1ms per division.

As you can see in the scope photo on the left, the damped oscillation is very nice.  The peak-to-peak voltage is about 400 volts and the ringing frequency is right at 20kHz.  The dwell of the drive to the coil is 1.2 ms.

On the scope photo on the right, we see the waveform at the drain of Q3, the negative half which is being effectively blocked by D6.

Note that I've added another transistor type in addition to the IRF840 (500 volts).  The IRFBE30 (800 volts) is less likely to have it's forward voltage rating exceeded and should be more reliable.

Remember, if you want to run this circuit off of a 12 volt battery, just remove T1 and BR1.  Leave C1 for stability.  Connect the positive battery lead to C1+ and negative to circuit ground.
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19 April 2017:
I decided to build a couple more of these ignitions to permanently mount on engines.  While doing so, I re-worked the design of the "fire-on-break" circuit.

Simple "Fire-On-Break" schematic with improvements.
I've made some simplifications and improvements to the original "Fire-On-Break" circuit.  One thing is that I added R6 which isolates the signal part of the circuit from the power part, improving stability.  That allows me to leave out the 1N4007 diodes.

Also, I've added a 10 Amp 1000 Volt P.I.V. diode to remove stress on Q2 and improve operation of the coil.

Lastly, an LED and driver has been added so we now have a "FIRE" light.

This ignition has been installed on The Non-Rotary Valve Engine along with cheap ($12) "dry" coil for a Toyota or Suzuki.  After about an hour of running, it is doing fine.

Suzuki coil.
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