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Please help with simple pulse generator

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averagef250

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I don't know much about electronics, maybe enough to scrape by. Hoping somebody here might be able to help. I'm building a fuel injector flowbench and cleaning system and using the Megasquirt ECU to actuate fuel injectors as if they were in an engine for flow testing. I need to generate a signal that will mimic the signal the ECU gets from the negative (distributor) side of the ignition coil. I need to be able to run the injectors the equivelent of idle speed to 10,000 rpm the pulse from the distributor would be 1/2 crankshaft speed so I need a device to generate 200 to 5000 pulses per minute for the ECU signal or 0 to 5000 will work fine. If anyone would like to check out the megasquirt: https://www.bgsoflex.com/megasquirt.html
I should ad that the flowbench will run off 115V ac and a regulated 12V DC power supply will power the megasquirt and high pressure fuel pump. A pot that I could turn to adjust the frequency would be the way to go.
I am capable of building a circuit from a schematic if someone could help me out with a circuit that would work.
Thanks for any and all help and happy holidays!

Dustin
 
Can you find out what the input pulse should look like? I have found several examples such as this one, and they all look similar. It would be possible to create a reasonable facsimile of this, but it may be unnecessarily complex. If you can find out from the mfr what the simplest workable pulse waveform looks like, it would help a lot. I'll bet they test it with a simple pulse generator. We just need to know required pulse amplitude, width, and the load imposed by the Megasquirt.
 
The way I see it the negative distributor terminal connects to the distributor. Inside the distributor either points or an electronic module open and close the negative side of the coil to ground. I would think all I have to do is switch the tach input to the megasquirt on and off to ground. Maybe "pulse" isn't the correct nomenclature for what I'm looking for.

I was also wrong on the number of pulses necessary. For some reason I was thinking one pulse every other crank revolution, but in reality, for an 8 cylinder application there would be 4 pulses every revolution of the crank. So in order to fake the megasquirt into operating at up to 10K RPM I need to generate up to 40K pulses per minute.

So I need a device that will switch the tach input to the megasquirt on and off to ground at a rate variable between 2000 and 40K times per minute. Thanks for the help.

Dustin
 
OK, if you think that will work, here's a circuit that generates a constant 1ms wide 12 volt pulse, independent of frequency. The frequency is variable from below 2,000ppm to about 40,000ppm. I suspect the pulse is too wide, but you can change it as shown. If you do, you may want to also change R2 to keep the maximum frequency about the same.
This doesn't generate a switch closure, it generates a pulse, which is what happens at the coil terminal. My concern is that the waveforms I saw when I searched showed pulse amplitudes of around 40 volts, with a 12 volt pedestal following the pulse, but maybe a simple 12 volt pulse will work. This is why I suggested you contact the mfr first. You may be wasting your time and mine by not doing so.
 

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Thanks alot for the help! I'll give it a try. What does VCC on pin 4 mean? The manufacturer sells a testing device that simulates the tach input for the megasquirt, but it's $40 and considerably more complex than I need. Thanks again!

Dustin
 
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