VSS signal not being recognized?

[bobski]

Originally Posted by glug

I finally settled on an lm339 based comparator

339, or 393? Though it's a pretty basic comparator circuit that should work with many op-amps, your diagram says 393.
10uF might be overkill for the decoupling cap on your input. Also, electrolytic caps are polarized, and I think the polarity is backwards in that diagram. Try turning the cap around and/or a mylar cap (or something else unpolarized in the fraction-of-a-uF range) and see if that helps with limitations you're seeing.

[glug]

Thanks again, Bob!

I couldn't find a 393 locally, so substituted the lm339 from rat shack. It seemed a reasonable substitution. And yes, the polarity for C1 on the posted schematic is wrong, and it is correct in my circuit. I should have corrected that before re-posting it.

I'll try a smaller cap, thanks for the suggestion. I can see how that might be a factor.

[glug]

I replaced the electrolytic with a .1uF polyester film...

When generating a controlled sine wave input, mpguino VSS pulses track well between 30hz to 130hz while the LCD is removed. With the LCD plugged in, there seems to be some additional noise in the VSS.

Below 30hz, there are spurious pulses. That isn't a huge concern, since my VSS is 10hz per MPH. Above 130 hz, I start to lose pulses. So, barring other ideas, it seems the next step is to carve the TTL circuit out of my somewhat spaghetti perfboard.

75mph is 750hz... So I do need to significantly improve the situation.

Fwiw, the injector pulse figures I see while in the truck look good.

[bobski]

Can you set up a 555 circuit (or a function generator if you have one) to give you a nice, clean square wave? It would be nice to rule out software limitations.

[msc]

The LM2907 is a low cost frequency to voltage converter that works also works great for interfacing to a variable reluctance transducer. It comes in a couple variations. The 8 pin part has all the signals necessary for this application.

Search digikey.com for LM2907 then download the data sheet. At the bottom of page 12 in the data sheet there is an example circuit for this application.

Mike

[bobski]

Originally Posted by msc

The LM2907 is a low cost frequency to voltage converter that works also works great for interfacing to a variable reluctance transducer.

Good call.

[glug]

Thanks for that suggestion, Mike. That looks like a good solution. Oddly, I had saved that data sheet back on may 7, 2009 - probably for similar reasons.

I still need to follow Bob's advice and verify the TTL measurement capability of my circuit. I don't have a signal generator, so I intend on driving it with another arduino and the tone library. Just haven't found the time, and I needed to take a break from it for a while.

[bobski]

Originally Posted by glug

Just haven't found the time, and I needed to take a break from it for a while.

I hear ya. I got one of those rack-of-bins things from harbor freight just to keep all my projects sorted out. I work on one until my attention wavers or I run out of free time, then it (and any associated parts) goes back in its bin until I feel like working on it again. Before I started sorting it all out like this, my bench was constantly 6" deep in cables, half-disassembled electronic devices, bare PCB boards, stripped wire insulation, clipped component leads and half a pound of assorted hardware. I would constantly set my safety glasses down and lose them in the clutter. Not buried or anything... Just hidden in plain sight.

[glug]

Originally Posted by bobski

Before I started sorting it all out like this, my bench was constantly 6" deep in cables, half-disassembled electronic devices, bare PCB boards, stripped wire insulation, clipped component leads and half a pound of assorted hardware. I would constantly set my safety glasses down and lose them in the clutter. Not buried or anything... Just hidden in plain sight.

If only someone would invent cordless dvm probes, I'd be free of their tangles...

Of course, then I wouldn't be able to find them by following the leads.

[glug]

I finally have an LM2907 in hand, and I'm ready to make this thing work... I'd like to cut down on the experimenting, so suggestions are appreciated.

The application note shows two circuits which seem to be appropriate (reproduced below, figure 23, from the top of page 14):

http://www.national.com/an/AN/AN-162.pdf

Any thoughts on which of those would be the best match for the MPG's VSS input?

The first circuit outputs pulses at 5v, and is also a frequency doubler (two-shot). The other other is a one-shot, and outputs a square wave of about 2.5v. Another gotcha - digi-key did not have the through hole 500nF cap needed for the first circuit. I have a 1000nF.

My VSS is about 10 pulse/mph, for 36K pulse/mile. So 70 mph is 700 Hz. Would 1400 hz be too high for the MPG input?

Since my proto mpg perf board is now a bit cluttered, I am planning on building this on a small sub-board.

Thanks!



From the AN:

Digital Interface
A growing proportion of the complex control systems today
are being controlled by microprocessors and other digital
devices Frequently they require inputs to indicate position
or time from some mechanical input The LM2907 can be
used to provide zero crossing datum to a digital system us-
ng the circuits illustrated in Figure 23 At each zero crossing
of the input signal the charge pump changes the state of
capacitor C1 and provides a one-shot pulse into the zener
diode at pin 3 The width of this pulse is controlled by the
nternal current of pin 2 and the size of capacitor C1 as well
as by the supply voltage Since a pulse is generated by each
zero crossing of the input signal we call this a ``two-shot''
nstead of a ``one-shot'' device and this can be used for
doubling the frequency that is presented to the microproc-
essor control system If frequency doubling is not required
and a square wave output is preferred the circuit of Figure
4 can be employed In this case the output swing is the
same as the swing on pin 2 which is a swing of half supply
voltage starting at 1 VBE below one quarter of supply and
going to 1 VBE below three-quarters of supply This can be
ncreased up to the full output swing capability by reducing
or removing the negative feedback around the op amp