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El Cheapo II DIY O2 analyzer success - Franken-analyzer is alive!

Discussion in 'Do It Yourself - DIY' started by dberry, Jun 17, 2015.

  1. dberry

    dberry Hydrophilic ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Philadelphia
    [For anyone doesn't know, OxyCheq sells a DIY kit to build your own O2 analyzer for ca. $110, which includes the Teledyne O2 sensor and all the parts you need except a 9V battery. ]

    Here's the link:

    OxyCheq - EL Cheapo II Analyzer Kit

    and here's a photo of my finished analyzer. Since I modified the design a bit, let's call it the new (and improved?) "OxyCheq El Cheapo II v2.0." (The main "mod" is the case. The kit comes with a generic cheap black plastic box, which I ruined, and replaced with a waterproof box with clear plastic top.)


    Here's a dramatic rendering of my own trials, tribulations, and ultimate triumph over stupid mistakes.

    The kit is pretty simple: a sensor, generic plastic case, an LCD display on a little PCB, some short lengths of wire, 1 resistor, a pot (poteniostat), on-off switch, battery clip, and input jack for the sensor. There are several pages with reasonably clear instructions. I opted to pay an extra $19 for a "10-turn pot" for finer control when calibrating. The "Tees" they sell to limit gas flow during sampling just look like PVC pipe fittings, so I'll just roll my own if I decide I need one.

    Step 1 is to cut holes in the top half of the plastic case for the display, switch, and pot. A drill is necessary, and a dremel is handy for this, but I also proudly pulled out my finely honed woodworking chisels to chop perfect rectangles for the LCD and switch. Every piece fits in the top half of the case snugly - yippee!

    [Note to Oxycheq: Adding a paper template for laying out the holes to the kit would save folks a lot of time and effort.]

    MISTAKE #1: I put the top and bottom half of the cases back together and notice the battery compartment in the bottom of the case interferes with the perfect holes I chopped in the top of the case for the switch and pot. Bad planning on my part. Now cheap generic plastic case is ruined; need new case.

    Step1B: Google "plastic project boxes for electronics" and find a gazillion options for a few bucks. INSPIRATION:

    Replace ruined, but cheap plastic case with a heavy-duty waterproof case with a clear plastic cover so the LCD is visible without needing to cut a big hole in the cover (thus better protecting the guts from the salty spray on dive boats.) Just drill holes for pot, switch and jack on the sides so the clear lid can be opened and closed without disturbing any wires.


    I ended up getting a 155x90x50mm weatherproof box with clear cover for about $8. (Boxes were only $2.50 on eBay, but I didn't want to wait 10 days for delivery from china vs. 2 days from a supplier with a warehouse in the US of A).

    Step2A: Decide my big-ass soldering gun is WAY too big for working on the LCD PCB. Order decent soldering pen with 5 different micro tips, coil of solder, and a solder-sucker. Thank-you Amazon Prime for free next day delivery.

    Step2B: Attempt to follow instructions to remove resistor R3 from PCB.

    Step2C: Dig around in my basement for a lighted magnifying glass so I can FIND resistor R3 on the PCB. [no joke, I could not have managed this project without a magnifying glass and light.]

    Step2D: Continue attempt to follow instructions to remove resistor R3 from PCB.

    MISTAKE #2: I didn’t fully desolder resister R3 before “flicking” it off the PCB, thus ripping off one of the copper contact pads to which it was soldered.

    Okay, I can follow the instructions and solder one wire from pot to the top pad, but where do I connect the other wire from the pot with the bottom pad totally MIA (and no trace of the trace)? [Note: R3 is ca. 2mm x 3mm, and each pad is 2mm x 2mm - i.e. very small for old eyes and my fat fingers!

    Step2E: I decide R3 and R2 were connected in series, so I solder the wire returning from the pot to a pad connected to R2. This seemed to give a working analyzer, so the recovery from Mistake #2 is complete. Move along, nothing to see here.

    Step2F: Solder the rest of the wires following the directions, being very careful to not screw up anything else!

    Small Modification: rather than soldering the leads directly to the lugs on-off rocker switch, I soldered on clip ends so the switch can be disconnected at the lugs while testing different arrangements of pieces inside the enclosure.

    Step3: drill and cut holes for jack, pot, and switch in the new case.

    Step4: fashion mounts for LCD from bottom of case by epoxying two 2inch long machine bolts to the bottom of the case. These pass through the holes in the LCD for the orignal bezel (which I discarded). On each bolt there is one nut to set the height of the LCD, a plastic bushing (1/4inch long tube cut from a Bic ball-point pen shell), and another nut on top of the LCD to secure everything. This turned out to be a really sturdy way to mount the LCD from the bottom; I can post a photo with details, if anyone wants.

    Step 5: Secure the 9V battery… hot-glue a velcro band to the side of the box, wrap band around battery. I can shake the analyzer and it doesn’t budge.

    Step 6: screw everything in place, bolt down the clear plastic lid, and turn it on.

    Step 7: turn pot until LCD reads 20.9 (in air).

    Step 8: Find a tank previously tested and labeled EAN 32.5% and crack open the valve: LCD on Franken-analyzer reads 32.4. That’s a Bingo!

    It's may not be 100% waterproof (although I could seal the pass-throughs with hot glue), but it should more than enough for normal use on a boat. And very rugged.

    I'll post some other photos showing how I mounted the LCD from the bottom, or of the case originally supplied in the kit (with very pretty, but badly placed cut-outs), if anyone is interested.

    I hadn't soldered anything involving electronics in years (just plumbing), so this was a fun little DIY project for me. And now I've got a nifty little O2-analyzer.



    Last edited: Jun 17, 2015
    Max Speed, undrwater and Diver Dan 28 like this.
  2. NAM001

    NAM001 Contributor

    # of Dives: 500 - 999
    Location: the moon
    you got a great kit and the ten turn pot is well worth the cost. It will run forever. Have fun
  3. jwelburn

    jwelburn Contributor

    # of Dives: 200 - 499
    Location: Oahu
    I love the clear case! Noted for when I build mine.
  4. Diver Dan 28

    Diver Dan 28 Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Oklahoma
    Ordered my kit this evening[emoji3] got the ten pot as well. I am hoping to put the sensor inside the box, does anyone know if that will work? Looking forward to a fun "science experiment" I'll post a review as well but I doubt my black box will turn out as nice as the OPs.

    Sent from my SM-N910P using Tapatalk
  5. dberry

    dberry Hydrophilic ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Philadelphia
  6. Diver Dan 28

    Diver Dan 28 Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Oklahoma
    This is going to test alot more than my nitrox tanks, namely my intelligence. Please tell me that there are more than 4 pages of instructions and a bad copy of the circuit board....
    Made the cuts on the black case and am happy with where everything is. Now trying figure out where all the wires go lol

    Sent from my SM-N910P using Tapatalk
  7. OkByMe

    OkByMe Contributor

    # of Dives: 500 - 999
    Location: Collierville, Tennessee, United States
    Hey guys... go to ebay and look for used Miniox 3000 oxygen monitors. I bought one new for under $200. These self calibrate and are accurate. They are used in mines and are approved for same. It works better than the Analox EII and you don't have to fuss with potentiometers. The replacement sensor is always 60 bucks and the one I use on my compressor has been going over 2 years without a sensor change. I test it occasionally with 50% and it's spot on, whereas the Analox is fussy about temp and humidity and the sensor lasts 6 months and cost over 100 bucks.
  8. Diver Dan 28

    Diver Dan 28 Instructor, Scuba

    # of Dives: I'm a Fish!
    Location: Oklahoma
    Finished mine this evening. This was a fun project.[​IMG]

    Sent from my SM-N910P using Tapatalk
    tridacna likes this.

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