Building an inexpensive Oxygen Analyser is an achievable home project thanks to the Oxygen Hacker’s Companion by Vance Harlow. The analyser I built is a variation with one major difference – the panel meter.
WARNING: From here on this blog gets seriously nerdy - you have been warned!
Aside from the O2 sensor, the panel meter is the major component of the analyser. After waiting three weeks for a PMA128A panel meter, my order arrived with the panel meter and a few other items cancelled. I also ordered a CX101A panel meter for a future more-compact analyser (but a CX102A was delivered instead - LED display instead of LCD). So my future project became my only choice.
As background, these 3.5 digit panel meters measure 0 to 200 millivolts (mV) (actually 199.9mV) using a 7106 chip or 7107 chip (basically the same chip but it drives LEDs directly). Electronic components are manufactured to a tolerance (e.g. + or – 5%), so to get an accurate reading the panel meter has a fixed resistor and potentiometer or ‘pot’ so that the builder can compensate for component variations. This works by using the pot to adjust a reference voltage (Vref), which is then used by the chip for comparison to the voltage being measured.
Adjusting the pot changes Vref and the display by a few percent. However, an Oxygen sensor won’t create 20.9mV for air, so a more drastic change is needed to Vref by replacing the fixed resistor. I tested my O2 sensor (a PSR 11-39-MNB from TFM Engineering) and it read 12.2mV in air and close to 59mV for my test O2 cylinder. So in this case the panel meter Vref needs to be adjusted so that 12.2mV is displayed as 20.9. In addition, the voltage from the sensor can vary with age and so too will Vref as the analyser’s battery gets low – so on-going calibration is needed. For further information refer to the Oxygen Hacker’s Companion.
As the CX102A uses the same chip as the PMA128A, so I figured I could apply the same theory and a bit of experimentation. After tracing the circuit, the resistor I needed was located near the on-board pot, it’s the surface mount resistor 'R3', labelled 9100 (910 ohm). R3 was removed using a soldering iron, tweezers and a magnifier – it’s only about 2mm in length. Tip: don’t have the soldering iron on the board any longer than necessary as heat will damage the circuit card and chip.
Next I soldered hook-up wires to the circuit pads either side of where R3 was – one on an original pad and the other to the side of the on-board pot as this was easier. As the soldered connections are small I glued the wires to the board with a hot glue gun.
The pot on the panel meter itself was set at 200 ohm, while the resistor R3 was replaced with a 500 ohm trimmer pot (a small blue square pot) and second larger pot for on-going calibration (with a knob on the front of the analyser) - see circuit diagram. The large pot should have been 100 ohm but I couldn’t find one, so I used a 1000 (1k) ohm logarithmic pot with a 120 ohm resistor soldered across two terminals (one in the middle and on one side) as this creates a fairly linear 120 ohm pot - close enough.
With a 9V battery, switch and some hook up wire, this was all tested on a project board using a separate circuit to simulate the sensor output range to be measured. The battery leads were soldered to pins 1 and 2 on the circuit card, and a join was made from pin 3 to 4 for the decimal point. The sensor leads connect to pins 7 (+) and 8 (-).
Once tested, everything was soldered together and assembled inside a plastic project box. With the goal of a ‘compact O2 analyser’ I used a project box for a TV remote control from Jaycar. A mono headphone socked was added to the box, and a headphone plug to the wires from the sensor, so that the sensor could be connected and disconnected as required.
I tested the complete unit, adjusting the blue internal trimmer pot to calibrate the analyser before closing up the box – minor adjustments are now made using the larger pot with the green knob on the front of the analyser.
All up the analyser cost about $150, with the sensor being $115 and the panel meter and project box about $10 each. It took about 20 hours in research, prototyping and assembly but the next one will be quicker.
A final note of thanks goes to Stephen from TFM Engineering for the sensor, sensor holder and Oxy Hacker’s Companion.