Electronics help... esp with a diode.

[The Chairman]

Partial Solution achieved. I finally traced the problem to a wire off of the 5k pot. It was showing actual resistance and I could feel a slight bump about a third the way from the circuit board. I replaced both of the wires and the analyzer works in and out of the enclosure. I also made it so I can easily take the board out and in without disturbing the pot by adding a couple of pin connectors. I went ahead and paired them male/female on each side. It's just a pot with no polarity, so I guess I didn't need to, but it was as easy one way or the other.

However, I still can not get the prototype to work. That's kind of disappointing, but at least I can measure my tanks now. I like the idea of having a back up or two and being able to easily replicate it gives me a bit more control over technology. So, at least I fixed it even if I can't reverse engineer it.

 

[lowviz]

...//... I still can not get the prototype to work. ...//...

OK, if you are still game, time to see what does and doesn't work. On page four of the datasheet, middle of page (toward the left), is a diagram for how to wire the panel meter for a single-ended input voltage. Rewire the breadboard that way and connect the sensor across the two inputs (INH & INL). The device should respond to various EANx's but will not read out in proper units as it lacks scaling resistors.

 

[The Chairman]

Yeah, that's the next thing I'm going to do. But I have the other unit assembled, so I am going to wait for the new panel meter to come in. This way I can have my analyzer and use it too.

 

[wedivebc]

Yeah, that's the next thing I'm going to do. But I have the other unit assembled, so I am going to wait for the new panel meter to come in. This way I can have my analyzer and use it too.

I built a rebreather PO2 monitor using parts from this guy Details of PPO2 Displays years ago. If he is still in business Mark makes good stuff

 

[mrfixitchapman]

I always say "If it ain't broke, keep working on it." Really. I always say that.

DC

 

[lowviz]

I built a rebreather PO2 monitor using parts from this guy Details of PPO2 Displays years ago. If he is still in business Mark makes good stuff

There is a nice schematic in that link. That would be the next step. Go from single-ended input to floating inputs and goof with the internal reference so that the display gives numbers that directly correspond to %O2.

 

[Zung]

Thanks lowviz, now I get it: the sensor delivers a known output voltage that's proportional to the O2 content.
The rest is just a simple millivoltmeter to measure that voltage; such a meter can be bought ready made for a few bucks, shipped to your door, as in here. I don't think single-ended or floating would make any difference in this case.

 

[The Chairman]

I just got some parts in... I might try to suss it out this afternoon on the breadboard.

 

[DiveNav]

So, who has a great oxygen analyzer circuit they would like to share?

Get a new analyzer

For a second I was thinking to send you (for free) one of our Nitroxbuddy DIY kit .... but then I remembered that at DEMA you said you do not like the idea of using a smartphone to analyze a tank :depressed:

Let me know if you changed your mind about that.

Alberto (aka eDiver)

 

[The Chairman]

Playing with it is far more fun Al!

I got the PM128 the day before yesterday. I also got a 1K large potentiometer and a 100K trimmer pot. The meter is not the 128B, but the current model. That means it doesn't quite have the layout as the one in Vance Harlow's Oxy Hacker book. I gueesed that R2 had to come out instead of R3. In retrospect, I guess I should have breadboarded this, but I went ahead and hard wired it instead. It works just fine. A $12,50 panel meter and $2.50 worth of pots and walla. My only problem with it is that the adjusting pot is way to sensitive. I actually got two of everything, so I think I'm going to breadboard the second one and substitute some different value pots in there. I still don't understand the circuit completely. It looks like I have three pots in series. I solder to the middle leg of the PCB mounted pot to one leg of the surface mounted pot. The surface mounted pot's middle leg is then connected to the middle leg of the other trimmer pot which I soldered to the far leg to the PCB and super glued it in place. When I first turned it all on, I got the "1" I got with the other system. A quick twist of the surface mounted pot brought up other numbers and I was able to adjust it to display 20.9. I put it on a tank of a known quantity and it replicated the number written on the tape.

So, I'm thinking that my breadboard with the LASCAR 200p is simply Out of Range. It's the same thing that happens with my DVOM. If I am testing a 12V circuit on the 2V range, it simply reads 1. If I turn the dial to the proper range which would be 20V, I get working digits that make sense. I'm not sure why it didn't occur to me the other day, but it's obvious at this point.

Looking at the back of the PM, the right side is all about powering up the system and the left side is all about measuring. Yes, the BL (Back Light pin) on the left is powered by the right side, but it's still not part of the sensor circuit.

The Oxy cell connects to INL and COM. The COM leg of the oxy cell has the 5K pot attached to it which is connected to the 1.5Kohm resistor and then to RI on the bottom left. I guess there has to be an opamp integrated into this meter and my PM is out of range because of it. I think I'm going to power up the meter and plug in only the Oxy cell. I wonder if I'll I see 11.2 milliamps. That's what I got with the PM 128 before I started modifying it.