I'm trying to build an EMF meter that is able to sense the electromagnetic field emitted by an electronic ignition system in car. I have built Aaron ALAI's AC EMF meter (EMF Detector - Aaron ALAI), but unfortunately it isn't able to pick up the frequency of the field emitted by a car. I'm guessing the frequency of the EMF emitted by a car is around 100 Hz (Depends on the RPMs, but for an idling engine.) I have considered purchasing the Tinkerkit Hall Effect sensor (http://www.tinkerkit.com/hall/), but I'm assuming that will not be sensitive enough as well. If anyone has had any experiences, or even any guidance, please let me know!
You may be able to detect the ignition signal on the vehicle loom if you are clever about it, but the usual way to detect it would making a direct connection is an inductive coupling on an HT lead. If you're allowed to make a connection then sensing it either from an RPM signal output by the ECU, or by detecting the pulse on the coil LT circuit. If you connect to the vehicle circuit then make sure you protect your Arduino against electrical spikes because a running engine produces a lot of electrical noise.
It needs to be "wireless." So I can't hardwire anything to the actual system. The Arduino and components have to be housed entirely outside of the engine compartment without any cabling going inside; therefore, I'm looking into an EMF detector. But thank you for the idea!