I am currently working on a DIY quickshifter. This stops the spark for a very short time when changing up gears so that gears can be changed at full throttle.

I have a working circuit, but it currently has no protection from load dumps or transient voltages.

The quickshifter will be controlled by an Arduino microcontroller. When a gear change is detected the Arduino will stop the coils from sparking for a number of milliseconds.

Here is a basic circuit of how the coils are fired.


The red wire connected to the Pri pin on the ECU supplies a constant 12V to the primary coils. Once a signal is recieved by the relevant PNP transister the 12V feed is stopped momentarily and this causes a spark to fire.

I plan to cut the wire feeding the primary circuit of the coils between A and B and then connect these wires to my Arduino circuit. This circuit will be powered by another wire, labelled C above.

Here is the circuit I want to add to the motorcycle.

enter image description here

I need to add some protection to this circuit to protect the MOSFET and MOSFET driver. The datasheet of the MOSFET driver advises to use a bypass capacitor but doesn't mention the size I would need to use.

Please can someone recommend some improvements to this circuit in order for it to be protected when in use on a motorcycle.

Thanks, David

  • 2
    \$\begingroup\$ Consider reading this apnote, which I found easily: ti.com/lit/an/snva681a/snva681a.pdf \$\endgroup\$
    – jonk
    Commented Sep 13, 2016 at 0:29
  • \$\begingroup\$ So the spark fires when the PNP transistors turn off. And your plan is to, effectively, turn all of them off at the same time. So, when you do that, you will cause all of the spark plugs to fire at that instant. This does not seem like a good idea. I don't know if a single misplaced ignition spark can damage the engine, but it wouldn't be good for it, I think. Wouldn't it be better to just bypass the existing PNP's with transistors of your own? \$\endgroup\$
    – user57037
    Commented Sep 13, 2016 at 2:04
  • \$\begingroup\$ The PNP transisters are integrated with the coil sticks. There is no way to access them. I do have access to the wires though where I can add extra circuitry. densoautoparts.com/direct-ignition-coils-cop \$\endgroup\$
    – Rickerman
    Commented Sep 13, 2016 at 9:35
  • \$\begingroup\$ It looks like you have access to all the emitters (that is where you plan to put your switch). And the collectors are connected to individual wires, aren't they? That is all you would need to bypass them. Anyway, I am just pointing out that when you turn off your switch, you are going to possibly cause an instant spark on all four plugs at the same time, irrespective of piston position. \$\endgroup\$
    – user57037
    Commented Sep 14, 2016 at 3:38
  • \$\begingroup\$ I have added some capacitors to my schematic that will hopefully smooth out some of the transient voltages. I am unable to edit the original question as I cannot post more than 2 links. \$\endgroup\$
    – Rickerman
    Commented Sep 14, 2016 at 15:29

1 Answer 1


Wondering if/how you solved this. I agree w/ mkeith above. You don't need to kill the 12V to the coils, you need to disallow it's interruption by bypassing the system transistors. You could do this by feeding 12V thru one large(er) MOSFET to all coil 12V inputs - thru diodes (so the coils don't crosstalk.) It would likely create a puff of black smoke and/or a tailpipe backfire. You're dumping raw fuel/air out into the exhaust and then firing fully (air/fuel)loaded cylinders after that. The flame will leak out of the exhaust valve and ignite what's' in the exhaust pipe. Sounds like noisy fun if you don't have mufflers to worry about. Backfires (exhaust explosions - not intake) will usually split the muffler depending on the construction. You may also want to blank out your injectors during this time. Injector blanking means lean cylinders but no spark to overheat... but power recovery might be a problem as the cylinders need to be reloaded with fuel once normal operation is restored (clutch released.) Your control switch could be a basic contact switch on the clutch. This could also be completely analog /w no micro(Arduino)...and thus no software...and thus faster. If you add the injector blanking function, you can always use it, or not. Injector blanking would involve denying drive signal to each injector. If they are individually fired you need a series switches(MOSFETs), one for each injector. If they all fire at once, then just interrupt power to all of 'em due to their only running on one channel, will suffice. If you're on carbs, your out of luck. ...Good luck.


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