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lefterisgaryfalakis:
Wait... Can I play this trick using this reed relay instead? Keep in mind that it is NO and I don't want to buy a NC since that will cost me 15 EUR...
Doesn't the magneto fire the spark plug? What is the peak voltage? I'm guessing that the reed relay (and most other small relays) will spark over at the voltages needed to fire a sparkplug.
johnwasser:
Doesn't the magneto fire the spark plug? What is the peak voltage? I'm guessing that the reed relay (and most other small relays) will spark over at the voltages needed to fire a sparkplug.
No, no, it doesn't! At least I wouldn't play with such voltages if it did! 
Actually it is connected to a step-up transformer which in turn produces the few kV required for the sparksplug to fire. The wires I want to tap to are before the transformer and produce a maximum of 9VAC (at least according to my DMM) but I suppose there are a few peaks but this is easy to mitigate.
If you could give a link or some details of the ignition you are using it might be easier to help.
Any weedeater or simular engine I have worked on, and it's been a lot of them, have all the ignition components in one module. Never seen on with any kind of step up transformer.
detown:
If you could give a link or some details of the ignition you are using it might be easier to help.
Any weedeater or simular engine I have worked on, and it's been a lot of them, have all the ignition components in one module. Never seen on with any kind of step up transformer.
I'm using this engine if that does provide you any info:
It might as you say not use a step up transformer but it doesn't supply a high voltage on the wire I'm tapping into. FYI I'm using the wire where you would normally plug a kill switch. I have no idea of what ignition it's using. I'm wrong, maybe it's a CDI (so probably a capacitor instead of transformer that I said)? But it has (afaik) the same purpose.
Is it possible to rectify the other half wave (which if shorted kills the motor) and then use an optocoupler connected to a MOSFET to create the short circuit?
Also, I've been suggested the attached schematic but for me it sounds like a recipe for disaster as there is no isolation!
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A relay on the kill switch....done.
Manual shows the kill switch as "12" which will be one cable switching to ground to short the magneto.
bluejets:
A relay on the kill switch....done.
Manual shows the kill switch as "12" which will be one cable switching to ground to short the magneto.
My first and most obvious thought.
Excessive current draw, especially when you are battery powered and the engine is producing a whopping 100mA at best...
Hi,
What other conditions have to be set for the Arduino controller to allow the engine to start?
Tom... 
lefterisgaryfalakis:
My first and most obvious thought.
Excessive current draw, especially when you are battery powered and the engine is producing a whopping 100mA at best...
No idea where or how you can imagine the current is "excessive" ........
Basically, you're talking through your hat in my opinion.
TomGeorge:
Hi,
What other conditions have to be set for the Arduino controller to allow the engine to start?Tom...
Just readings from various sensors combined with timers. I have not decided the exact conditions but whith some fine tuning I will figure them out later on.
bluejets:
No idea where or how you can imagine the current is "excessive" ........
Basically, you're talking through your hat in my opinion.
Yeah but compared to the 100-150mA that my engine is producing it's precious to say the least! 
It is more than enough for my needs, but a relay is a deal breaker here!
Any other solution is welcome though (except latching and ssr which are unavailable where I live)!
Thanks!
A lot of assumption here. That wire is not any kind of generator wire.You are not going to charge a battery with this. It simply shorts the primary side of the ignition coil to ground to kill the engine. If you think there is no high voltage on that wire, here is a little test.
With the engine running grab that wire with one hand and touch the engine block with the other.I think you will have a shocking experience.
If you would give a comprehensive explanation of what you are trying to accomplish it would be a lot easier for people to help you.
Not to be derogatory, but from the comment you made about being too young to drink coffee, I would assume that you are a youngster diving into something without doing the proper research.
detown:
A lot of assumption here. That wire is not any kind of generator wire.You are not going to charge a battery with this. It simply shorts the primary side of the ignition coil to ground to kill the engine. If you think there is no high voltage on that wire, here is a little test.
With the engine running grab that wire with one hand and touch the engine block with the other.I think you will have a shocking experience.
If you would give a comprehensive explanation of what you are trying to accomplish it would be a lot easier for people to help you.
Not to be derogatory, but from the comment you made about being too young to drink coffee, I would assume that you are a youngster diving into something without doing the proper research.
I'm well aware of the fact it's not intended to supply power. Just think of it as a hack or in other words what this forum is all about...
It does indeed give you a shock but the possibility is very slim and this has happened to me only once (even when touching the engine with bare hands). Anyways, the current is so small that (to my understanding) it should be possible to smooth it out for charging a lead acid battery.
Not to be derogatory, but from the comment you made about being too young to drink coffee, I would assume that you are a youngster diving into something without doing the proper research.
Yeah, while it's true that I'm only 15 years old, I think that what's a better way to learn engineering other than doing projects like this? Building this on a simulator would get me nowhere as close! And basically I have done quite a lot of research. I mean enough to discover that I'm connected before the CDI.
What I'm trying to accomplish (taken from the original post):
I want the engine to start ONLY if it receives an ok signal from the Arduino.
If that wasn't enough:
There are 2 terminals coming out from the engine with an AC output. Using a half wave rectifier and appropriate smoothing, it is possible to charge a battery with it. I'm using a half wave rectifier because the 1st half wave is used for the ignition (CDI) and if I place a full bridge rectifier I have to limit the current to not turn off the engine by accident but using a half wave rectifier, I can use that single wave that isn't used for the ignition and therefore doesn't affect the engine in any way. The other wave that is used for the ignition, if shorted, turns the engine off. Looking for a way to short it without using relays (which draw a lot of current and I can't afford that) or SSRs (that are unavailable in my country).
Hope this sums things up...
Not quite.
You might notice that there are not many people replying to your post. The reason is people get tired of trying to drag out information, like pulling teeth.
A rundown of the entire project.
Why do you need to charge a battery?
What is the Arduino going to do?
What information are you sending to the arduino?
What information are you sending out of the Arduino?
What is the 2 cycle engine going to do?
Who or what is starting the engine.
And what do you mean "I'm connected before the CDI." I don't think you have a clue about how these cdi modules work
If you would explain what you are trying to do with the complete project it would give people a chance at giving you a cohesive answer.
The secret to getting good answers is asking good questions.
And your statement "what's a better way to learn engineering".
Engineering is based on facts not assumptions.
Not trying to be an ass, just giving a little advise on how to get help in solving your problem.
And...
Why can't you drink coffee at 15?
-jim lee
detown:
Not quite.
You might notice that there are not many people replying to your post. The reason is people get tired of trying to drag out information, like pulling teeth.
A rundown of the entire project.
Why do you need to charge a battery?
What is the Arduino going to do?
What information are you sending to the arduino?
What information are you sending out of the Arduino?
What is the 2 cycle engine going to do?
Who or what is starting the engine.
And what do you mean "I'm connected before the CDI." I don't think you have a clue about how these cdi modules work
If you would explain what you are trying to do with the complete project it would give people a chance at giving you a cohesive answer.
The secret to getting good answers is asking good questions.
And your statement "what's a better way to learn engineering".
Engineering is based on facts not assumptions.
Not trying to be an ass, just giving a little advise on how to get help in solving your problem.
Hi there and sorry for replying late... Your points are actually correct and I will do my best to mitigate the most of the issues you described in your very comprehensive post.
- The reason I didn't give that much info initially is because I don't think that is really required as that not only would make the post look like a giant wall of words, but would confuse you even more without really any benefit. However since you point this out as being a cause I will do my best to respond and provide more information on the subject.
- As I said, the engine is the one you can see in the attached photo, a few posts earlier. It's a 25cc, 2-stroke, pull-start, weedeater motor. Nothing special. If you have ever seen a weedeater (which you have :D), you probably know what I'm talking about.
- Initially I didn't feel like making a rundown of the entire project as this all seems irrelevant (to me) as to how that would affect finding potential ways to shutdown the engine.
What is the project supposed to do?
The project is a DIY bicycle-like vehicle that you are supposed to drive. I've finished it 1 year prior to writing the initial post and so far it's been quite funny but lacks the specialized controls I want (because remember, I'm an electronics guy after all ;)).
What I'm going to add on the project?
I want to add a speedometer that I've created specifically for the bike (Arduino-based) which will communicate with a DIY ATtiny84A-based BMS (which will charge my lead acid battery) via USART. Long story short, I want the BMS (as the speedometer will be turned off in power saving mode) to provide an "everything ok" signal or the engine won't start. Also, if the engine is running and midway a problem occurs the engine should be turned off. That all, allows for anti-theft features, protection features e.t.c. that I want to incorporate. A relay is just NOT a viable option because of it's excessive power draw.
Battery's main role and charge process:
As I mentioned earlier, there will be a 12V SLA battery (1.2Ah) that will act both as a buffer for my charging solution and as an energy reservoir so the ATtiny84A can power up and give the ok signal to allow the engine to be started (by pulling the rope).
The ATtiny will control switching on and off the charging process as well as my load (speedometer, lights, e.t.c.) as well as killing or preventing the engine from starting as I mentioned earlier.
As to how the battery will be charged? No space for solar panel and the need to produce power even if the vehicle is stopped (dynamo therefore not suitable). Since the engine is running and can produce a current proven to charge my battery I'm fine with using a half wave rectifier. No high voltage spikes or anything since I'm using a boost converter and the battery. Not to mention that the engine's current is not enough to raise the voltage that high. Also, found out that only the cycle I'm using for turning the engine off produces the high voltage. Well, as you said I have not understanding of how a CDI works and this is all assumptions, but (seems to) work in practice. Haven't killed any board so far and I've connected Arduinos, MCUs and even charged my phone a dozen times (to test the capabilities of the system).
Bottom line: This was just intended to be a fun project that would teach me a lot about electronics. If I forgot to mention anything please let me know. Anyways, really appreciate your help and efforts to "get me out of the mud"!!!
Cheers guys!
jimLee:
And...Why can't you drink coffee at 15?
-jim lee
I only drink water, milk and orange juice! Haha!!!
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