Electronics help... esp with a diode.

[The Chairman]

Now I know why the circuit didn't make sense to me. I have poor eyesight. I have been basing the circuit for this on the runners I could see, While I was cooking breakfast, what I had read/saw on the data sheet kept coming back to me: INH was not being utilized. So I brought my breakfast down to the grotto and did an continuity check between the header connectors for the PM. Sure enough, INH and COM are connected and so are RI asnd RO. I looked through my illuminated magnifying light and had to add a jeweler's loupe to see the runner between the header pins. I quickly added two jumpers to my breadboard and it works! Yay! OK, using the SP200 will cost about twice as much as the PM128b, but you won't have to desolder any surface mount resistors to get it to work. Also, while the numbers are plenty readable even for these old eyes, the SP200 is a far smaller package, enabling it to fit in a smaller enclosure. I wonder what diameter/thread pitch these oxy cells are? I have students showing up in just a bit, so I'll be heading down to the grotto to get ready for their arrival. I'll probably have to wait till later before I suss that portion of this project out.

 

[Zung]

Would help if you sketch a little schematic.
You can use a 20 turn trimpot to reduce the sensitivity.
Logically, the sensor should be connected to INH & INL (input high & low).

 

[The Chairman]

Here is my schematic for the Lascar SP200. I'm going to change the 5k pot to a 10k pot.

 

[Zung]

If you can get a rough setting with the 5K and your only complain is it's hard to zero in on the exact setting ("sensitivity"), changing to a 10K with probably make it worse. I've got a 5K multi-turn (20?) right here, I can send it to you tomorrow if you give me your address. Should get there in a week.

 

[The Chairman]

OK, I got Zung's 5K trimmer and a 10K trimmer and a 10K 270 degree sweep pot. All work fine. As I was switching between pots, I noticed that the meter returned to 11.4, which is the reading I get off of the oxy cell alone with my DVOM. Obviously, there is an op-amp integrated into the meter. When I measure pure oxygen with either meter, I get only 96.4. Good enough? Probably. It could be the way I am sampling it. It could be the cell. It could be the overly simplistic way this analyzer adjusts the oxygen cell. I think it should be better.

My cousin Bill sent me this link: How to choose A Gas Sensor - Wiki It has an Arduino friendly CO sensor for $7.50. I've got one on order.

Special thanks to Zung for sending the trim pot!

 

[Wookie]

It's all Greek to me....

 

[The Chairman]

It's all GEEK [-]Greek[/-] to me....

I fixed it for you... But that was way funny.

 

[DiveNav]

.....It has an Arduino friendly CO sensor for $7.50. I've got one on order.....

... this makes me wonder why in the hell Google is spending $3.2B for Nest Lab's CO sensor .....

 

[Mars67]

I am really enjoying this thread. I am also in the process of learning about electronics and love DIY projects.

 

[Mars67]

So how did the $7.50 sensor turn out?