I have a Radio Shack frequency counter (#22-305), with two frequency ranges and a digital pulse mode. Its input is a 50 ohm BNC connector.
Can someone suggest a suitable inductive pickup for this to wrap around a spark plug wire that will provide a signal that is not too strong?
My first impulse was to use a a few feet of RG-58 with a BNC at one and and a foot of insulated wire on the other end soldered to the center conductor, fitted with an alligator clip to clip to the exposed cable shield after wrapping the spark plug wire a few times.
There are many ways to pick up the impulse from the spark plug. Your way will grab some signal. You will then need to condition the signal for your counter. You will need a scope to look at what you are getting and shape it to the needs of your counter.
Use 1 loop of wire in a very small radius. Use a pair of diodes like 1N914's to clamp the input at the 2 connections. Each diode goes the opposite direction across the connection.. You may also want to use a 47 ohm resistor in series with the loop feeding the center connections. the diodes should be clamped at the center connection. this will give you an average of 0.6 volt +/- pulse to the input of the counter..
all you should need to do is just lay the loop next to the wire.
It's a crude method but I've done a coupling like that before to detect RPM.
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The frequency of an engine at highway speed, maybe 2500 rpm, is 41.66 Hz. The frequency of firing of a single cylinder is 1/2 of that (four-cycle engine), or about 20.83 Hz. If you can pick up a signal for all cylinders, as on the center of the distributor cap on an older car, you would get 125 Hz for a six-cylinder engine. According to some web references, the frequency counter you have has a lower frequency limit of 1 MHz, so it probably won't work. Even if your frequency counter could see it, for such low frequencies a period measurement might be more practical, if you invert and scale it for rpm.
Actually, it might not be. Depends on the ignition system in use, if it's a Distributorless Ignition System it could be using wasted spark where the coils are double ended and fire two cylinders (one firing, one exhaust stroke) at a time, hence it fires twice per revolution. It could also be multi-spark in which case you're screwed. Of course it might not even use an ignition coil if it's a lawn-mower engine or similar!
On a DIS coil pack you can get a very usable signal from the cables back to the ECU from the coil-pack, I've used 12 turns of roughly 26 SWG ECW on a toroidal ferrite core using a couple of diodes and resistors to clamp the signal as 'Jamie' describes and had excellent results.
The best results I've had were using a hall effect device (get one from a dead DC brushless fan) on one of the injector bodies feeding a simple comparator (LM393). Of course this will also work on the body of an ignition coil if the car doesn't have fuel injection or you just can't get at the injectors (single point injection)
Period measurement with averaging and smoothing (you have to reach a compromise though if the engine can change speed quickly, think bike engine) is the best way to go, even if the frequency counter can measure that low the gate time will be a killer unless there's a PLL to lock in (then you have problems with jitter because internal combustion engines are rarely stable from one revolution to the next for a myriad of reasons)
If the OP is just trying to measure RPM it doesn't have to be a spark plug signal, there may well be a top dead centre signal from the camshaft or crankshaft or at least a crank position sensor that outputs some kind of pulse train (usually an inductive sensor and a pattern formed into the flywheel with a missing pulse to indicate some period before TDC)
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