Inductive pickup?

[bruce]

Can anyone tell me what kind of output an inductive pickup on a timing light puts out? (before I destroy one in the name of R&D)

My latest undertaking is a chassis dyno attachment to hook my engine dyno to and I need a way to get engine rpm, currently I am picking it up off the dyno shaft and this will not work for a chassis dyno setup.

A chassis dyno setup will let me test a whole car/tractor or use my standard engine dyno framework to test just engines. My goal is to be able to roll a tractor, microsprint, kart or any small wheelbase vehicle onto my dyno and test it. I've had a few people ask me already about being able to do this so I am looking into what it would take.

Thanks in advance.

[Jesse Lackman]

Bruce you might want to check with the makers of the LM-1 wide band O2 meter, they have an inductive pickup that is made to use with the LM-1 for an RPM signal.

$20

here is is;

[86rocco]

Bruce, I don't know if this will help out any but the output signal from an inductive pickup is similar to an audio signal from microphone, the simple circuit shown below from the , allows the output from the pick-up to be recorded on any standard audio device for later analysis.

[Tony]

Bruce, these ignition lead pickups operate on the current transformer principle, so the output is going to be a very fast current spike when the plug fires. The output from the pickup coil ABSOLUTELY MUST have a fairly low value resistor connected directly across the output winding. The commercial pickups will most likely already have this fitted inside.

The output voltage will be essentially zero, with a very rapid and narrow pulse first in one direction, then the other, returning to zero fairly quickly soon after the plug has fired. By itself this is going to be a fairly unfriendly waveform to work with, and other equipment may not work too well off it directly.

But, the output waveform will look very much like that which comes out of the pickup coil used in an electronic variable reluctance distributor.

I have not actually tried this, but perhaps the simplest "off the shelf" signal conditioning system might be an ignition module or igniter salvaged from an electronic distributor. It would be well worth a try.

These ignition modules are used to seeing input signals that look very much like the signal that should come out of an inductive ignition pickup. The igniter will also stretch and amplify the very narrow pulse, to generate the necessary stretched "dwell" angle.

Just place a pullup resistor (say 100 ohms) to twelve volts from the output terminal if the igniter module, (where the ignition coil would normally go). That should then give you a nice clean 0-12v square wave output that can then be fed into some sort of tachometer circuit to indicate Rpm.

These ignition modules are good because the input signal can vary from just a few hundred millivolts at idle to tens of volts at redline, and swing in both directions. The module should work perfectly with some fairly nasty looking waveforms.

I suggest you beg, borrow, or steal an oscilloscope to get this working initially.

[86rocco]

Bruce I don't know what kind of budget you're on but the looks perfect for a small dyno application.

[bruce]

Well my budget is pretty small if none exisitent. I have so far in the past 2 weeks scored the signal conditioners off ebay. My strain gage conditioner came today and I'm waiting on my freg to volt converter this week. I had already picked up a thermocouple meter with +/-10volt output. So hopefully I will get my dyno up and running with some halfassed DAQ in the next month or so. Anything beats "manual mode"

With all the info I received here on the inductive pickup it confirms what I was thinking that my digital meter will accept the output the same as my current inductive pickup does. I do have a second digital tach so I will plan on adding that into my display console. I need to be able to have a simple rpm hookup for the chassis dyno I am planning on building for my eddy current brake. The spark plug inductive pickup solves that problem.

I would love to have a more complex DAQ setup but I have to go with what I can afford to do right now, I paid less than $100 for all the signal conditioning I purchased off ebay. Ebay is a wonderful place! Now I just have to get it all working . . .

[rab]

Bruce,

What are you going to build your roller out of, I am looking for idears to build a roller dyno to put 4 stroke karts on.

Rick