Spark plug wire inductive pickup for RPM 4

[ziploc]

This is kind of a repeat of a very old thread:

http://www.eng-tips.com/viewthread.cfm?qid=223731

I'm looking to read RPM on my paramotor (120cc two-stroke engine) with an Arduino. The part I'm particularly curious about is sensing the spark and conditioning that signal. As mentioned in the older thread, there are very simple tachs on the market that simply use a wire wrapped about 4 times around the spark-plug wire (with one end completely free). I've seen a few approaches to conditioning the signal, but it sounds like "itsmoked" had a slick approach that unfortunately didn't appear in the old thread. Hopefully he can chime in here.

Thanks in advance for any tips.

 

[pmrobert]

Is there an ignition cutoff switch? If so, you may want to grab a signal off that. It's still dirty with lots of ringing but should be cleaner than inductive secondary pickup. The low voltages you're seeing re "What I did find surprising is that it showed only about 3.7V when I was running the circuit from a decidedly tire 9V battery and about 4.75V when I was running the circuit from the USB cable from my laptop." may still need to be addressed. The display processors, in my experience, get unstable with dirty/low power although I do take note of the fact that all is well until the engine is running.

 

[OperaHouse]

YIKES........Where was that written. Good call on the power. I read a lot of posts at 3AM when I can't find my glasses. My eyes generally glaze over when a post is longer than three sentences. I would avoid powering anything external off the UNO on board regulator. Since this is an aircraft and I presume the design will be shared with others, I would avoid connecting anything "directly" to the engine like to the points.

 

[pmrobert]

Just a thought - In the interest of reducing RF, does that engine use a resistor plug? If not, see if they spec a resistor plug and/or plug wire option. Non-resistor plugs and wires emit enormous amounts of RF compared to the resistor versions. Also, relative placement of your device can make a huge difference - like any other EM radiation, RF follows the inverse square law - double the distance means 1/4 the amplitude.

 

[Mixotricha]

Hi guys ,

This is my first post here so please bare with me a moment. I have read through this whole thread and I have a need to build either a capacitive or inductive rpm counter for a small two stroke engine. I am going to use an Ardunio as the micro controller and am quite happy with that on the code side. I am going to power the Ardunio off a separate 9 volt battery. This is not going in a flying machine and most of the time this unit wont be turned on so a small battery to power the Ardunio is fine.

This two stroke has one of the combined CDI/Magneto units. This is housed in a magneto cover on the side of the motor. It has the usual grounded kill switch configuration. One wire comes out of cdi/magneto unit and goes to kill switch. Other wire from kill switch comes back and is grounded to motor.

I don't want to build a tach in this instance but just a simple rev/cut that kills the motor above a certain rpm. So I will be counting pulses and at a certain number of pulses killing the motor via the cut off switch circuit.

What am I interested in is the last post about being able to read the spark plug pulses off the kill switch wires?

Is the pulse in the kill switch wires due to the design of these ignition units or is it just inducted noise?

If I hooked up for example a speaker across the kill switch wires would I hear an audible click from the plug?

Just trying to get my brain around how to use that signal

 

[pmrobert]

Were it I, I'd characterize what exactly is on the kill wire. It could be anything from logic level easy to manage signal all the way to 300-400v from the CDI coil primary - and that's from a largish capacitor, not merely coil flyback. The mid 80's Yamaha dirt bikes used the last scheme - you gained much respect for that kill wire when working on those machines. Needless to say, if it is the coil primary you're going to have a tough time taming that to logic level. Not impossible though.

 

[Mixotricha]

I had a look today with a MM and I see that the kill switch circuit hovers around 8V at about 60Ma on full revs so I have decided not to take a signal off that after all.

Instead I set up an Ardunio with one of the freetonics hall effect sensor modules.

http://www.mixotricha.com/2013/02/11/simple-hall-effect-test/

Wrote a simple bit of code that counts pulses driven by interrupt outputs to serial monitor.

Much to my surprise the hall effect seemed to pick up the pulse in the ignition lead just fine. I was planning to put the hall effect somewhere close to the stator but I think somewhere on the ignition lead might just do it for my purposes.

This is what I have so far. Super simple.

http://www.mixotricha.com/2013/02/11/where-to-put-the-ground/

The only thing I have not done yet is to ground the Ardunio to the frame. Sometimes the Ardunio just locks up and I need to unplug it and plug it back in again. I presume this is because I have not grounded it to the bike frame anywhere.

Before I do I just wanted to see if people thought my approach was a reasonable one? It seems almost to easy!

 

[Mixotricha]

I had another brain wave. Would it be possible for me to just put a resistor in parallel across the kill switch circuit to achieve what I want to do?

I just want the motor to cut out when it gets above a certain RPM. I was thinking maybe just an adjustable pot?

 

[pmrobert]

A multimeter is not going to help characterize what's on that kill wire. You need to scope it. More often than not there are short duration high voltage spikes on any conductor close to or connected to an ignition system - especially CDI. You write "It almost seems too easy!". Historically and anecdotally it's not easy to get a stable, accurate microprocessor-safe signal from an existing system. The processor inputs are very easy to fry, don't be surprised if you end up with inputs that don't work any more if you connect directly to any ign system wiring or even try to do an inductive coupling scheme.