picking up signal from car's ignition coil - Electronic Circuits Projects Diagrams Free

cbraga · 19th October 2005, 05:37 AM

I'm trying to build a tachometer from the electromagnetic noise emitted from a car's ignition coil... I've had some success in separating the signal from the noise but I'm kind of stumped...

After some experimentation with an oscilloscope it seems I get the clearest signal using one loop of wire, half-inch diameter. I'm connecting only one end, since it seems the signal becomes very weak if I ground the second end, and plugging it into the circuit like in the attached figure. I place the coil 5 to 15 cm from the car's spark plugs and connect it to the circuit via shielded cable.

The signal in question is made of very high frequency spikes which happen every time a spark plug is fired. On the scope they're so quick I can barely notice. I'm having a hard time because if I try to make the antenna more sensitive (winding more loops into the coil) it attenuates the spikes and they get lost among the noise... If I make it less sensitive the signal will be too weak to work with...

I did found another circuit using a similar approach, here: http://www.redcircuits.com/Page23.htm Unfortunately that configuration didn't produce good results for me...

What do you think? Is that a reasonable setup or is there some antenna or circuit configuration that could yeld better sensitivity to this signal?

Someone Electro · 19th October 2005, 05:43 AM

Did you try puting an coil close to the ignition coil to detect the changing magnetic field of the coil being swicthed on and off.

cbraga · 19th October 2005, 02:38 PM

Quote:

Originally Posted by Someone Electro

Did you try puting an coil close to the ignition coil to detect the changing magnetic field of the coil being swicthed on and off.

Yes... I don't think it made a lot of difference though.

zachtheterrible · 19th October 2005, 11:59 PM

I wonder if a hall effect sensor would work better?

evandude · 20th October 2005, 01:57 AM

well most tachometers that I have seen use the 12v part of the ignition coil to trigger from... with a zener diode and some caps/resistors for input protection and all, you can get a usable signal out of it...

or are you specifically going for non-contact sensing?

cbraga · 20th October 2005, 02:41 AM

Quote:

Originally Posted by zachtheterrible

I wonder if a hall effect sensor would work better?

I wonder too... Most sensors are of the on-off type, so I probably would end up with a different sensor for each car. And linear sensors are hard to obtain (for me anyways).

Quote:

Originally Posted by evandude

or are you specifically going for non-contact sensing?

Not necessarily non-contact, but at least to avoid digging and messing with the car wires...

Today I ordered an inductive sensor of the type used in ignition timing lights and automotive multimeters for the exact same purpose... we'll see if it helps..

evandude · 20th October 2005, 02:47 AM

you should try wrapping a wire around the wire going from the ignition coil to the distributor and seeing what kind of signal you can get... it will probably be a whole lot better than what you get just by holding an antenna near it.

cbraga · 20th October 2005, 03:30 AM

Quote:

Originally Posted by evandude

you should try wrapping a wire around the wire going from the ignition coil to the distributor and seeing what kind of signal you can get... it will probably be a whole lot better than what you get just by holding an antenna near it.

That's a very nice idea. In fact, I can't believe I haven't tried that! Thanks!!

zachtheterrible · 20th October 2005, 05:35 AM

Another idea is a microphone to hear the RPM.

I think pike did that to measure the RPM on his lawnmower with his oscilloscope. Or maybe it was someone else.

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19th October 2005, 05:43 AM	(permalink)
Someone Electro	Did you try puting an coil close to the ignition coil to detect the changing magnetic field of the coil being swicthed on and off. __________________ Il give you shocking experience.

19th October 2005, 11:59 PM	(permalink)
zachtheterrible	I wonder if a hall effect sensor would work better? __________________ I'm no electronics god, i just talk too much.

20th October 2005, 01:57 AM	(permalink)
evandude	well most tachometers that I have seen use the 12v part of the ignition coil to trigger from... with a zener diode and some caps/resistors for input protection and all, you can get a usable signal out of it... or are you specifically going for non-contact sensing? __________________ EEgeek.net

20th October 2005, 02:47 AM	(permalink)
evandude	you should try wrapping a wire around the wire going from the ignition coil to the distributor and seeing what kind of signal you can get... it will probably be a whole lot better than what you get just by holding an antenna near it. __________________ EEgeek.net

20th October 2005, 05:35 AM	(permalink)
zachtheterrible	Another idea is a microphone to hear the RPM. I think pike did that to measure the RPM on his lawnmower with his oscilloscope. Or maybe it was someone else. __________________ I'm no electronics god, i just talk too much.