Analog PO2 Display

DanFountain

I've just posted a first look at my PO2 bar graph display. http://www.portup.com/~dfount/rbmain.htm
This came out of a discussion with Bill Sewell, Tom Rose, Dr. Bob and a few others on the rebreather list some time ago.

Comments?

-Dan

Sutty

Looks nice in normal use, but confirming cell function by dil flush harder.

__________________
Never forget that life is a finite resource.

Stephane

Care must be taken when using LM324 circuits: I found that when used whith low input voltage, the linearity of the transfer function is not maintained across the whole range of the O2 cell.
Instead, use a LT1014CN from Linear Technology or any low noise op-amp.
This was with a non inverting gain of around 2.

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Stéphane

rdmmdr

nice clean layout
why dont you try the max494 for the op amp they will send them as samples for free
rick

Scuba_Vixen

Another vote for a better choice of op amp.

The 494 mentioned is pin compatable and a *major* improvement.

One thing I would question is if this is for your main PO2 readout, or just a quick glance approximation, similar to what a HUD is for?

If it's for your main PO2 monitoring, I think it's wayyyyyy too lacking in resolution, as in unacceptably so, at .2ata per increment. Something on the order of 0.05 ata should be a max between increments.

If it's for a quick reference of "OK", or just a bit high or low, (where you also have a good digital readout) then expanding the scale so that each led is 0.05 PO2 and ranges from 1.05 to 1.5 would make it quite usefull.

From a visual perception standpoint, you may want to build one up with each readout in the horizontal plane, with 3 rows, as opposed to 3 columns. It would give you quicker recognition of which led's ordinal position is lit, and let you get a more accurate reading, quicker.


Please take this constructively, I'm not trying to just be picky.



Darlene

E-man

Gotta love maxim and their free samples...plus they will send you a 'catalog' everynow and then filled with some amazing chips

DanFountain

Thanks for the suggestions - even "picky" ones!

An upgrade to the opamp is definitely in order - the MAX494 and LT1014 are considerably superior to the LM394 as far as the input offset is concerned. Sure enough, Maxim is sending me a couple of free samples. The MAX294 has a maximun Vcc of 7 volts, so I'll have to add a battery holder for 4 AAs instead of a single 9 volt - not necessarily a bad thing, as I haven't checked the battery life of the current 9 volt setup.

Expanding the scale of the bargraph is a matter of adding a couple of resistors - all three chips could use the same reference system. The chips could also be stacked, giving a 20 segment bar graph - but also doubling its size. Would definitely have to go to horizontal rows in that case.

The bar graph can be operated in either dot mode or, with the addition of a jumper, in bar mode. Any opinions on which would be more visible or more useful? For example, having the lower segments turned on might make it more obvious that a yellow LED is a high warning rather than low. On the other hand, in dot mode a quick glance would tell the diver if it's all green or not all green.

Dan

Scuba_Vixen

Stay away from the bar mode.

1) It'll be majorly confusing ...... We tend to associate the bargraph with quantity, but not where the desired quantity is a portion in the middle.

2) More importantly, Let's look at current draw, .... with only the 3 leds in dot mode lit, figure 15 to 20ma each, plus some for the chips, so it's a minimum of ~50ma. 9V batteries are maybe 150mah for a rechargable NiMh. A 3 hour max runtime for a new battery is not acceptable. That's a new one or a recharge for every dive. That means you have to open the unit and seal it back up every time.

Now imagine the current draw with 6 or 7 leds for each display lit, you're in the 400+ ma range. ... Just not practicle.

If you're really wanting to have some type of graphics driven visual display, you might consider a microcontroller with a graphic mode LCD display.

http://www.parallax.com/detail.asp?product_id=27936

If you want something brighter, you could use a VFD instead.

http://www.parallax.com/detail.asp?product_id=27970

but those puppies suck up some power. Figure almost 300ma for such a setup. ... Pushing the limit of practicality.


Having recently built some custom PO2 monitoring and HUD electronics myself, I entertained these same display ideas wanting something unique and different.

I have to tell you, after prototyping them all and watching them run in simulation mode for hours as I work at the computer here, I concluded that 3 lcd panel meters arrayed vertically, was the fastest, easiest, and most "reading error resistant" display of the bunch.

The only thing that came close, was the VFD, with the PO2 numbers across the top from left to right, and 3 horizontal rows with a dot mode lit 6X8 segment display. .... And it was totally impracticle from my point of view for what it was needed for.

Keep us posted on how it goes, and take me with a grain of salt if you wish.



Darlene

solocavediver

Suggestion: when you do the Mark 2, why not make the following simple improvement (requires an extra 1/2 of an LM324 or better). Arrange that the voltage across your chain of diodes varies (in a sawtoothy way) by just under twice the voltage required to switch from one position to the next. If you're bang on 1.0 (say) the thing will steadily read 1.0. If you're slightly above 1.0 it will mostly read 1.0 but the 1.2 led will gently start to flash. Once you're above 1.1, the display will spend more than half it's time reading 1.2 but continue to slip back to 1.0 occasionally until you've almost hit 1.2. Then, you get a steady 1.2.
Simple way of increasing the resolution?? Vary the mark/space ratio...

Cheers,

Charles. (I go for LCDs myself. Have broken several small multimeters in the process...)

Joseph Grech

IMG_0757.JPG

IMG_0759.JPG

IMG_0762.JPGDarlene,

What do you think of my vertical setup. I know it is not the most good looking but it works. Also in the other pics is my diy hud and how I fit 3 R22D into a p port with some help of my beloved black tape.


Joseph