loudhvx, look inside

As long as you use a very short tube from the manifold to a pin-hole restrictor, and then to a long, or larger tube to connect to the sensor, you will get average MAP data. The restrictor with the larger volume act as a dampener, which gives average-MAP.

Here's some info regarding manifold vacuum and port size etc, and why to use restrictors. I've posted it before. It's for manometers, but the info applies to reading MAP as well.
home.comcast.net/~loudgpz/GPZweb/Manomet.../ManometerNotes.html

RE: 3.3v microcontroller system.

I don't think that schematic I gave will work. The signal won't go much below about 1.7v. That may be too high for the input to register as low. The reason it's so high is that igniter's don't bring the coil signal all the way to ground. It's always somewhere between .5 and 1v depending on the system and how much current is flowing (which changes with time during the dwell period).

Let me see if I can think of an easy transistor circuit to work around it.

MAP doesn't really correlate directly to slide height on a CV, or even on a mechanical slide. MAP is a function of RPM, throttle position, and slide height (on a mechanical carb, the slide and throttle position are directly connected, of course). That's a lot of variables to juggle.

For a mechanical carb, you really only need TP and RPM to make a determination for the jetting based on AFR.

With a CV carb, the MAP would help, but a slide height sensor is what you'd really want, in order to know where to set the needle. That's a tough gadjet to make... unless you can just set up the video camera with a time code. That would be cool.

loudhvx wrote: RE: 3.3v microcontroller system.

I don't think that schematic I gave will work. The signal won't go much below about 1.7v. That may be too high for the input to register as low. The reason it's so high is that igniter's don't bring the coil signal all the way to ground. It's always somewhere between .5 and 1v depending on the system and how much current is flowing (which changes with time during the dwell period).

Let me see if I can think of an easy transistor circuit to work around it.

But if I split the voltages after that circuit, lets say 5 goes to 3.3, that means 1.7 will go to 1.2. That might work, no?

I found an ultrasonic sensor that would be ideal for measuring slide height, but I suspect it'd be out of my price range.

Would it be possible to embed a small magnet into a slide and use some sort of inductive sensor mounted to the carb body to detect slide position directly? If you machined a small hole and press fit a magnet in there, it wouldn't affect slide weight too much. Just thinking out loud.

Something like that might work Josh, I'll think about it some more. Good thinking.

As for my idea of splitting those voltages, wont work, the max low voltage for this board is 0.8.

Actually, I think your voltage divider idea will work, but the other components have to be changed a little. The zener will clamp to 9v, the diode will pull down to 1.7v, then the divider will divide by 3... so 3v high, and .6v low. As long as the micro's input is high enough impedance, it should work. No pullup needed on the micro.


EDIT: The above diagram is wrong, as pointed out by T_Dub.
The corrected one is below.




The 560 ohm R1 may have to be 1 watt or even higher, if the ignition is turned on while the crank is not turning.




(Also, I had to edit the spec on the first diagram. It should be 1 watt, not 1 amp on the zener spec.)

Cool, that looks good. Am I correct that all I would need is a zener and a diode to make the 12V off my kill switch into around 3V to pull her out of sleep? My kill switch isn't going to be attached to my ignition anymore.

I searched for some stuff about getting a magnet to sense linear position, and I found these cool honeywell sensors that would work, but I can't find anywhere to get them. The sensor is called the HMC1501. You need an array, but they're very accurate and could really do the job of sensing slide position.

Edit: Well I found somewhere to buy them, $20 each and I'd need 5 to do it. Screw that.

A 3v zener would work, but you can just use a diode and an LED to get the same results, plus the LED tells you it's on. I don't have a number handy for a 3v zener.

I've spent quite a few hours trying to think of ways to detect slide height directly. Nothing so far, at least nothing simple.

Yeah me too, its a pain. I can only think of the magnetic ones but they're so darn expensive.

In case the micro's input floats a little bit high, it'll need a pull down resistor.

Excellent, thanks.

I was thinking, it might take a modded carb cap for my BS34, but what if I stuck a slide pot above the slide, with a linkage up to it? I think that'd do nicely. If I could get a slide pot with low friction it could work I think.

Anyways, off camping now, I'll talk to you guys when I get back!