Homemade PO2 meter

Bnscherm

I think an LED HUD is going to be my next project so I sketched up the attached circuit.

Does anyone have hands on experience with the LM3914 ICs? I've planned to set the reference on the lm3914 for a full scale dot mode referenced at 2 volts. The opamp then uses a pot for variable gain for the calibration/setpoint. With a 2 volt reference that gives .2 volts (pp02) per line.

With 5 HUD LEDs the theory is that the user would see:
1. Red LED => 0 - .2 pp02
2. Yellow LED => .2 - .6 pp02
3. Green LED => .6 - 1.4 pp02
4. Yellow LED => 1.4 - 1.6 pp02
5. Flashing Red LED => 1.6 - 2 pp02

I'll be playing with this circuit next week to see if it works how I think it will. I'll post updates...

Attached Images

Hud.pdf (9.7 KB, 69 views)

whoizrob

Has anyone played with BI-color LEDs (Radio Shack part #276-012)? Seems like you could make a compact HUD and display loads of info with just two of those LEDs. If you ground one leg of the LED you get Red, the other Green. If you alternate your ground quick enough you get Yellow. With two of these in your HUD you could create following pattern using whatever PO2 values you please:

(R= red, Y= yellow, G= green)

RR- Flashing
RR
RY
YG
GG
GY
YR
RR
RR- Flashing

Seems to be enough resolution for most needs, no? It just seems unnecessary to me to have a large array of LEDS when you can discern HI PO2 versa Low PO2 just by the implied progression of colors left to right or right to left. I've yet to play with these LEDs since I switched jobs and all the fun hobbies have fallen by the wayside for the time being but would love to try it out with my Stamp PO2 meter project. Thoughts? The Picaxe looks pretty cool too... hrmmm.... might have to dust off the ole breadboard again =)

Rob

Joseph Grech

Hi, I have been diving my homemade hud for a couple of years driven with picaxe. They are very easy to work with, I have amplified the sensor voltage with LM324 op amp and use a varia resistor of 100K for calibration of the input voltage of the chip. I use picaxe18 and to get full redundace I have 3 different circuits with 3 different chips and powersupplies. I tell you it is a battery guzzler but i use cheap rechargable 9 V batteries. All is epoxied and tried. I can post pictures of my project.

Joseph

Bnscherm

Please do!

DanFountain

Take a look at the thread from a few months ago re: Analog PO2 Display http://www.rebreatherworld.com/home-...2-display.html. There's some good advice as to using precision op-amps versus the RatShack LM324.

You could simplify the circuit by not bothering to set the reference voltage - just use the internally set ref. Since you're boosting the cell voltage anyway, it doesn't really matter what the reference is.

I've attached a sketch of a very similar circuit to monitor 3 cells, with green being on setpoint, yellow being off setpoint but "safe", and red being "dangerously" high or low.

-Dan

Attached Images

PO2 HUD 1.pdf (27.1 KB, 77 views)

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cstmwrks

I've made three or four bar graph lm3914 PP02 meters. Round dial face, hud style and in line. Plus I have used it to trigger a timer for running a snap tite solenoid. I think the LM3914 chip itself is a winner. My trouble has been more with the op amp and have had most of my issues of reliability with that item.

A few variations involved using the bar graph in dot mode to form a simple voting logic. All it would take was any two segments to agree ( the segments are the pre defined set point ) and the signal went to a summing amp to trigger the solenoide. If a sensor went hi or low you would get a visual clue as to what one.

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Why you ask... well it's not like I have anything better to do.