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picking up signal from car's ignition coil

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cbraga

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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: https://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?
 

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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.
 
Someone Electro said:
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.
 
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?
 
zachtheterrible said:
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).

evandude said:
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..
 
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.
 
evandude said:
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!!
 
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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