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Sound Card Engine RPM Measurement
To measure RPM you need access to a Trigger signal that provides a known number of pulses per revolution. You can use fuel injector, spark, or crankshaft sensor signals. Important: See below for how to safely connect to your sound card.
Note that the Trigger Mode must be set to Normal, not the default Auto, when you are making RPM measurements. Auto mode is for "Automatic", not "Automotive". It's designed to show what's happening on the signal line even when triggers are missing or infrequent, so if a trigger signal isn't found in 20 msec or so, it "fakes" a trigger and updates the trace anyway. Those spurious triggers will inflate the RPM reading.
The Frequency Counter Cylinders (Cyl) control must be set to twice the number of trigger pulses per revolution. The default value is 2, meaning one event per revolution. That's appropriate for industrial equipment that provides one pulse per rev from (say) a photo-optical detector looking at a piece of reflective tape on a machine part.
However, each cylinder of a 4-stroke combustion engine only fires once per two revolutions, so if you are using a single injector or spark signal you should set Cyl to 1. (Leave it at 2 for a 2-stroke engine.)
If you are monitoring the coil wire or breaker points on a conventional distributor-type 4-stroke engine, set Cyl to the actual number of cylinders. (Set twice the number of cylinders for a 2-stroke engine.)
Set it to the actual number of teeth present on the crank position sensor gear. Don't count the missing teeth that are used to mark Top Dead Center.
Connecting to the Sound Card:
Sound cards typically expect signals in the +/-2.5 volt range. They may be able to handle more (maybe twice or three times that) without damage, but it's not worth the risk. Instead, you should use a simple limiter circuit to protect your card.
The standard 1 Megohm input circuit discussed in Input Range and Limiter Circuits is suitable for connection to automotive signals up to +/-27 volts. It will not be damaged by several hundred volts such as from the breaker points or coil primary of a distributor engine.
However, such levels will be clipped so that they appear to be in the +/-27 range. If there is ringing on the coil primary signal that exceeds that, each firing will appear to be a burst of pulses instead of just one, and it may be hard to get a stable trigger signal.
Instead of using the coil primary, you could use the first limiter circuit discussed under Input Range and Limiter Circuits (with R1 = 1100 ohms), plus an inductive sensor. For spark signals, this sensor is just several turns of wire wrapped around the spark plug or coil secondary wire. You can use a standard "mini alligator" cable, typically about 18 inches long. Build your limiter with studs or small wire loops to allow you to clip onto the input and ground connections after wrapping around the plug wire.
The induced spark signal will be small and narrow, but it is possible to get good RPM readings with this. However, the chosen plug wire must be located so that your sensor coil is not too close to other plug wires, or you may get double-triggering. You can usually see this by watching the waveform for smaller pulses. Besides moving the plug wire and sensor farther away, you can also adjust the Trigger Level so it only catches the highest pulses.
Hint: To make it easier to find the proper Trigger Level, you may want to switch to Auto mode during the adjustment. Hold down the SHIFT key and use the mouse to drag the horizontal Trigger Level line up and down on the waveform trace area. When the Level is too high, you will see pulses at random times across the screen. Drag it down until you start to see pulses aligned on the left side, keeping the Level line above any smaller pulses.
The above assumes your spark signal is larger in the positive direction than the negative. Otherwise, you can reverse the mini-gators to flip the polarity, or you can set Trigger Slope to Neg and work in the opposite direction.
For engines with fuel injectors, you can use the same limiter circuit as above. Since fuel injectors are typically just specialized solenoid valves, they generate large magnetic fields when they operate. These can be detected by an inductive sensor.
You may already have a suitable sensor within reach: Ordinary audio headphone drivers consist of a diaphragm attached to coil of wire that slides over a permanent magnet. These are essentially the same as dynamic microphone elements, and can indeed be used to pick up sound.
But as inductive sensors, actual sound isn't even needed: The magnetic pulse from the injector solenoid can directly induce a current into the earphone coil.
This effect depends upon good coupling of the magnetic field to the sensor coil. Ideally, the headphone element should be directly over the back end of the injector, so the coil in the element is on-axis with the solenoid coil. This is easy with on-the-ear phones if the injector is flat on the end... just hold the element against it. (Remove the foam pad from the headphone element first, so it doesn't pick up dirt and oil from the engine compartment.)
For other injector or phone styles you may need to do some experimenting. In-the-ear phones may be less sensitive than on-the-ear phones. You may want to rig up some sort of clip to hold the phone in position, especially if the injector is hard to reach or near the hot exhaust manifold.
If you have a "junk box" of spare electrical or electronic parts, try experimenting with other coils that you happen to have on hand, such as relay coils.
See also Automotive Applications
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