My Manifold Pressure Gauge - putting it all together

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Since I am a mechanical engineer, a lot of the thinking required for programming these AVRs hurts my head. Not literally, but I find I spend a lot of time wrestling with thinking through algorithms and getting confused and having to start over with the thinking process. Also, it takes me some time deciphering the syntax of C. I definitely feel like a fish out of water. :shock: My method of dealing with this is to take breaks and do things that are less intellectually strenuous.

I have made a lot of progress on the tachometer code but I have taken a few breaks along the way. Here is what I am doing for some of those "breaks"....

My first post with the AVR Freaks listed projects I want to make. The first item on the list was the tachometer I made in this thread.

The second item on the list is this Manifold Pressure (MP) gauge. Here is an image I showed in that first post...

When I made the bezel for the tachometer, I made an extra, since I knew I would be making this gauge. However I needed to make a different diffuser for the LEDs since the display is slightly different. Here are some images of the process...

I start with a piece of 1/2" (12mm) white translucent acrylic and machine away a pocket, leaving trapezoidal 'islands' that will be light pipes for the LEDs. The large rectangular island below is for the seven segment display area.

Since the LEDs are pretty close together and there wouldn't be enough space between the trapezoids for the cutter, I only cut every other one. I then make a separate part that fits on top and extends the remaining trapezoids between. The rectangular island is where the nomenclature "MP" is below the digits.

This is the middle, top, and bottom segments of the seven segment digits being cut.

The two parts of the diffuser "mold". After the vertical segments are added, the two parts will be cut out of the larger piece of acrylic.

This is the vertical segments being cut - note the pins left on top of each one.

The vertical segments are broken out. The pins fit into holes drilled in the bottom of the pocket with the vertical segments.

All segments have been added including a decimal point. Epoxy has been cast around them, along with pieces of aluminized mylar (from a fiber bar wrapper) to make an opaque barrier between segments (to minimize light 'bleeding' between segments).

You can see the perimeter of each part has been cut. There is still a thin section of plastic left that will be easily broken to release each part.

Slots are cut in the top plate to allow epoxy to be poured into mold.

The mold is assembled. Pins in the corners locate the top to the bottom and screws hold them together. You can see pieces of aluminized mylar inserted between each trapezoid. The holes around the perimeter allow bubbles to escape when epoxy is added.

A close-up of the aluminized mylar strips between each segment. You can see how close together they are.

Epoxy has been added. It bubbled up quite a bit when put in a vacuum. The diffuser for the tachometer didn't get the vacuum treatment and had several large bubbles in the finished product.

The bottom (back) of the 'mold' is machined to expose the epoxy and segments. Here you see slots being cut for the LEDs to protrude into.

The top (front) of the 'mold' is machined down to the proper thickness of the diffuser. The perimeter is then cut down to leave a thin section of plastic.

The finished diffuser is broken off the remaining parts of the 'mold'.

Since the two circuits are very similar, one of the PC boards of the tachometer is used in the MP gauge. The difference being, one of the PCA9922s is left off along with eight LEDs. Also, three jumper wires are added (fortunately there are vias in the PCB that allow these wires to be easily soldered in place).

The PC board was originally intended to allow some HP seven segment LED displays to protrude through a rectangular hole in the middle. Here you see a small PCB was added in this rectangular hole. Since this PCB was designed for the tachometer with four digits, the hole is larger than required for the three digits of the Manifold Pressure gauge. However, after beginning the construction of the diffuser, I realized that there was no provision for a 'units' display. You can see on the finished diffuser that there is a rectangular area of acrylic to the right of the digits.

Another small PCB (a section cut out from another unrelated PCB) is part of what fills the rectangular hole in the middle. This small PCB has a place to solder additional LEDs to back light "In Hg" as the units on the right of the digits. In the image below, you can see that the last eight 'slots' for LEDs on the arc are not populated. The seven segment LEDs have not been installed yet.

The diffuser, engraved face, and tinted lens have been assembled. You can barely see the "In Hg" nomenclature to the right of the digits.

The above PC board is the display board. Next I will populate the CPU board and then connect the two. After that, it will be back to software.... :shock:

Cris

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)

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Hi Cris,

I said it before, and I'll say it again: Wow! I am incredibly impressed!

JC

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DocJC wrote:
Wow!
Thanks Jay! I appreciate the nice words.

I just wish I had 1/100th my understanding of hardware in the software department. :?

Cris

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)

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I have found synchonous sampling for MAP works well in my engine management systems. The sampling rate is x times RPM. Accurate MAP is essential for speed density EFI.

A large x and equal weighing, provides sufficient filtering, using a moving window of samples.

There is a sampling limitation at high RPM, but sync of measurement is less critical there. The sampling rate saturates at fixed value.

It all starts with a mental vision.

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KitCarlson wrote:
I have found synchronous sampling for MAP works well in my engine management systems.
Thanks Kit, that's good to know. That would require an input from the tachometer, which is not part of the PCB layout (although a jumper could be added rather easily). Since this gauge is only for pilot information (and not part of an EMS), hopefully it is less critical so synchronization isn't needed. :wink: If it is needed, it looks like it can be added with little trouble.

Cris

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)

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For display purposes only there is more flexibility. I typically set up a timer ISR to read ADC, then trigger next reading. The readings are accumulated when count is reached, scale to desired units for display. The scale and display is triggered from a flag set in ISR, work done in main. The tach experience of update rate will help decide the number of accumulations. I often use 32 or another power of two. If less than 64, an unsigned integer can accumulate 10bit ADC results. Scale work often requires a long variable.

I forgot to say in my last post, your work is excellent.

It all starts with a mental vision.

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KitCarlson wrote:
For display purposes only there is more flexibility. I typically set up a timer ISR to read ADC, then trigger next reading.
Thanks Kit, good ideas. I like it. :wink:
KitCarlson wrote:
If less than 64, an unsigned integer can accumulate 10bit ADC results.
I plan to use a generic GM style MAP sensor....

The the range the gauge will display is from 0.0 to 30.0 In Hg. So, as far as I'm concerned, eight bits should be fine (especially since .1 resolution is more than necessary). Yes, I plan to use an unsigned integer variable.

KitCarlson wrote:
I forgot to say in my last post, your work is excellent.
THANKS! :wink:

Cris

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)

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I have now populated the CPU board.

You can see it was originally intended for off-the-shelf seven segment displays. Since I couldn't find displays that were as bright as the rest of the LEDs in the display, I made my own.

To replicate the off-the-shelf displays you can see I have a lot of 'legs' like what would have been on them. You can also see three jumpers (between the legs) that connect the LEDs that back-light the "In Hg" label.

Cris

P.S. I forgot to say that I also added LEDs for the seven segment displays....

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)

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Fine, fine work!

"Experience is what enables you to recognise a mistake the second time you make it."

"Good judgement comes from experience.  Experience comes from bad judgement."

"Wisdom is always wont to arrive late, and to be a little approximate on first possession."

"When you hear hoofbeats, think horses, not unicorns."

"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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My recollection (and a quick review on the net) tells me that manifold "pressure" is really vacuum (in a normally aspirated engine). And, the key action points are often quoted in "inches". I think I see a decimal point in your numeric display, XX.X.

Why isn't the manifold pressure given as a negative number? From my own curiosity, is the number absolute, showing (I would assume) 30" at sea level and engine not turning? Or is it differential (pressure below ambient)?

Thanks,
Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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joeymorin wrote:
Fine, fine work!
Thanks again JJ. :wink:
ka7ehk wrote:
My recollection (and a quick review on the net) tells me that manifold "pressure" is really vacuum (in a normally aspirated engine). And, the key action points are often quoted in "inches". I think I see a decimal point in your numeric display, XX.X.
Technically, manifold pressure is the opposite of vacuum. The measurement is taken from the same place on the engine, but the "zero" reference is at opposite ends of the scale. A vacuum gauge measures from atmospheric pressure as zero. Manifold pressure measures from a pure vacuum as zero.
ka7ehk wrote:
is the number absolute, showing (I would assume) 30" at sea level and engine not turning?
Exactly.

It is all arbitrary but has become standard in aircraft to reference manifold pressure.

The units are in Inches of Mercury. It is a reference to a manometer filled with mercury. One could also use water (inches of water in a water manometer), and that would give you a different set of numbers (just as one could use other units such as Bars, etc.).

And yes, the gauge displays inches and tenths of inches....

Cris

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)

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For MAP sensor I use MPXA4115. It is about the size of a penny. I place on board. It is also available in 2.5 Bar and right angle port.

Attachment(s): 

It all starts with a mental vision.

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KitCarlson wrote:
For MAP sensor I use MPXA4115.
I have a few Motorola sensors like that. I opted for the GM MAP sensor because they have been working in the harsh environment of automobiles for decades. I don't know if that is an issue or not, but hey, they're relatively inexpensive....:wink:

Cris

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)

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I have not experience a Motorola failure in numerous cars over an 11 year period. Accumulated moisture freezing, and rupturing the silicon die, is what often results in failure of OEM sensors used by GM and MOPAR. I had to replace one on a Dodge when I lived in the snow belt. Under hood environment is often a poor location for a sensor.

It all starts with a mental vision.

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KitCarlson wrote:
Accumulated moisture freezing, and rupturing the silicon die, is what often results in failure
KitCarlson wrote:
Under hood environment is often a poor location for a sensor.
Good information Kit. I assume you have your sensor in the cabin?

I've now finished all the hardware... both mechanical and electronic! :P 8)

Here is the electronics all assembled....

And the gauge all put together....

All that's left is software! :shock:

Cris

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)

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Nice package!

I have less faith in my design ability. I often make software first with rudimentary hardware. I sometimes learn improvements, mainly pin choices.

I like to use cabin when possible for locating EMS, but I have a larger secondary enclosure that is used for under hood use, in stock location on Porsche 914's. I do have an on board LM50 temperature sensor.

A sensor die can be protected by a flourogel coating. The problem I had was with TBI injection, mid 80's. It would have more moisture in the intake, during cool down condensing at sensor. It is possible the GM sensors now have flourogel.

It all starts with a mental vision.

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KitCarlson wrote:
Nice package!
Thanks Kit! :wink:
KitCarlson wrote:
I often make software first with rudimentary hardware.
I do it the other way around - hardware first with rudimentary software. :shock: Hopefully I'm learning to get past the 'rudimentary' stage with my software.... :?

Speaking of software...

Since the Tachometer and Manifold Pressure gauge are almost identical - hardware wise, I loaded the tach software into the MP gauge as a test. And it WORKED! :P :lol: :o :D 8)

I noticed that the 'units' label to the right of the digits was dimmer than I wanted (probably because the letters are so narrow compared to the rest of the gauge), so I swapped out the LEDs for some brighter ones. Now it's perfect! 8)

Here is a picture with all LEDs on...

The LEDs are so bright, they saturate the camera's sensor and make it difficult to get a decent photo.... :shock:

Cris

Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away. Antoine de Saint-Exupery (1900 - 1944)