Depending on where you get your parts this could cost you less than $120
I purchased all my parts at Futurlec (Other than the sensor) I find these guys very good on variety and price. Sensors in Australia start around $150, and can be as high as $220. I found Oxycheq have a Teledyne R-17, for US$80, with postage it cost me AU$93.40.
An alternative sensor available in Aus is PSR-11-39-JD available from DiveTek, I have not tested this sensor but specifications are very similar R-17 is 7 to 11 mV, PSR-11-39-JD is 8.5 to 14mV. Cost is $88 (June 2014)
(April 2016 Update)
I have found AquiferTec has PSR-11-39-JD1 for $120 delivered. I have tested this and works fine.
Oxycheq even have a DIY EL Cheapo II Kit for US$110. By the look of the parts it looks very similar to the below. with postage it would cost about AU$130.
Firstly you must modify the PM128A LCD Panel. The default scale for this panel is for a 0-200mv scale. We need to change the conversion factor of the PM128A to increment in 1mV for every 0.5mV change and then strap up a decimal point. To do this its a matter of adjusting the reference voltage on pin 36 of the LCD Controller chip. The formula for calculating the scale factor is:-
Vref = 1000 x (actual voltage/scale voltage) or 1000 x (10.45/209) 10.45mV Sensor reading of air, and 209 the voltage you want to see on the display. This equates to 50mV. (1000 being the internal clock period)
This is connected to R2 (30kΩ). See picture top left quadrant of the picture (Labelled 3002, its a surface mounted resistor). Its easy to find the pins of the controller chip. The chip is the blob in the middle of the board, the pins are in a standard 40 pin arrangement around it. Count from bottom left pin1, to bottom right pin 20, top right pin 21, top left pin 40.
R2 needs to be replaced with a value that will allow the scale to 27kΩ resistor and a 100kΩ multi turn trimpot. Then simply place a jumper on P1 and the decimal point is now before the last digit changing the reading to XX.X instead of XXX. I drilled a small hole in the side of the box to allow an adjustment and used superglue and hot glue to hold the trim pot in place
To connect the sensor to the panel you have to build a small calibration circuit. This is done using 1 x 100kΩ, 1 x 27kΩ resistors, and a 10kΩ Pot, configured as such.
A 9v battery is connected via a switch and a diode to the V+ V- on the panel. I put this diode in series to prevent a battery reversal from causing damage. Stupidly, not wearing glasses, I tried to put a new battery on and touched the wrong terminals. The panel is suppose to handle a battery reversal for a short period, but how do you define short. 2 to 3 seconds was to long....New panel required.
The sensor I used from Oxycheq has a normal reading for air between 7-11mV. (Nominally 10.45mV) This way with the above modifications the meter will read 20.9 for air. Each 1% of oxygen increases the voltage of the sensor by 0.5mV. or 1% on the modified meter.
All prices below are in AU$
Parts List Futurlec and Jaycar (20th July 2011)
|Item||Part Number||Price||Part Number||Price|
|Panel Meter||PM128A||$6.90||QP5570 (Not tested)||$27.95|
|Resistor 100kΩ x 10||R100K14W||$0.10||RR0620 x 8||$0.46|
|Resistor 27kΩ x 10||R027K14W||$0.10||27kΩ RR0606 x 8||$0.46|
|Multi trimpot 100kΩ||CERMR100K||$0.63||RT4620||$1.60|
|Diode x 10||1N4004||$0.20||ZR1004 x 4||$0.50|
|3.5 to 3.5 Cable||Not available||WA7006||$4.95|
The only other thing to get is a 9v Battery. Franklins or Coles $3.00
Once the build is complete you have to calibrate the unit.
Set the front control knob to midway. Set the blue trim pot on the back of the display panel to midway, connect a sensor (exposed to air) and then adjust the multi turn trim pot until you read 20.9
When a new sensor is used, re-calibration may be required, also as the sensor ages, the output voltage drops, again re-calibration may be required.
Mk2 has a diode mounted on the switch between the switch and the board.
Mk3 added the multi turn trim pot.