As a builder of gadgets and a scuba diver I have been trying to build an oxygen analyzer for a bit now. The big problem is that most micro controllers can’t read the mv that the sensor puts out. I played with opamps, but never got it to work properly. Finally I found the Texas Instruments ads1015, an analog to digital convertor chip “adc”. Adafruit makes a nice breakout board to make your life easier.
Now I only need a micro controller, an LCD screen, an oxygen sensor, a push buttons and like 9 resistors or so and I got yourself an oxygen analyzer.
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Step 1: Shopping List
Below are the components and parts with the links to where you can buy them
- Adafruit Pro Trinket 3v micro controller https://www.adafruit.com/products/2010
- Adafruit Lipo backpack charger for the Pro trinket with battery https://www.adafruit.com/product/2124
- Adafruit ads1015 breakout board https://www.adafruit.com/product/1083
- Nokia 5110/3310 monochrome LCD https://www.adafruit.com/product/338
- 7 10k ohm resistors
- 1 300 ohm resistors
- 1 push buttons
- 1 mono mini headphone plug
- Hookup wire
- Audio Jack - 3.5mm (Panel Mount) https://www.sparkfun.com/products/11154/
- Teledyne R17 O2 Sensor http://www.oxycheq.com/analyzers-sensors/sensors/...
- Sensor Cable *2 Foot* http://www.oxycheq.com/analyzers-sensors/accessor...
- Sensor Tee http://www.oxycheq.com/analyzers-sensors/accessor...
Step 2: Connecting It All Together
VDD Pin to 3.3v
GRD Pin to Ground
SCL Pin to A5 Pro Trinket
SDA Pin to A4 Pro Trinket
A0 Pin to Connected to the Audio Jack - O2 Sensor + pole
A1 Pin to Pull up Resistor 10K
A2 Pin to Pull up Resistor 10K
A3 Pin to Pull up Resistor 10K
VCC Pin to 3.3v
GRD Pin to Ground
SCE Pin to D8 Pro Trinket
RST Pin to D6 Pro Trinket
D/C Pin to D5 Pro Trinket
DN(MOSI) Pin to D11 Pro Trinket
SCLK Pin to D13 Pro Trinket
LED Pin to 200 ohm resistor to 3.3 V
1 pin to Ground
2 Pin to D3 and a 10k pull-up resistor to 3.3v
G Pin to Grd on Pro Trinket
3V 3V Pro Trinket
All the connections to the pro trinket are described in the above descriptions. I have added one 10k ohm pullup resistor to the RX pin. The Pro Trinket has the habit to sometimes hang on boot. This pullup resistor seems to remedy this. Why?..... Don’t know, a much smarter tinkerer on one of the Adfruit forums told me this piece of wisdom.
The Lipo Backpack is the power source for the project. Every 3v connection needs to be connected to the Lip Backpack 3v and all the Ground connections need to be connected to the Lipo Backpack Ground including the audio jack witch is connected to the Ground of the Oxygen Sensor.
Step 3: The Code
All the libraries used are from Adafruit and are easy to download from their site at www.adafruit.com. The formula for calculation the O2 % is simple.
(current mv from sensor / mv from sensor in the air) *20.9.
The only thing you need to do is store the "mv from sensor in the air" value in a variable before you try to analyze any gas. I do this in setup routine of my sketch or when you press the push button. Just download this zip file and upload the sketch to your pro trinket.
Step 4: Finishing Toughes
To complete the project I put all the components in an enclosure and added 5v Regulator like a the 7805T witch I connected to the 5V in pin and the ground pin on the Lipo backpack to charge the battery.
I also added some code to calculate the maximum operating depth “mod” of the analyzed gas for a PO2 of 1.4,1.5, and 1.6
Formula for Imperial “Feet”
mod= ((po2/fraction of gass)-1)*33
Formula for Metric “Meter”
mod= ((po2/fraction of gas)-1)*10;
I also added a button so I could create a menu to select your mod unit “Meter or Feet”. To prolong the battery life I made it so that you can turn the backlight on the LCD display on and off.
I could show you how to do these things but figuring this stuff out is half the fun. If you get stuck I can help a little. If you know how to improve on the project please let me know as it is always good to learn new things.