DIY Oxygen Analyser Parts List

Updated 28-4-2003

Back to Analyser Page
The Printed Circuit Board

Unfortunately, I based the original parts list on the Maplin catalogue, only afterwards discovering that they're a terribly unreliable stockist (their website is equally pathetic - or at least was), they no sooner begin to stock items than they discontinue them.
So far they've discontinued the 10 turn 50K potentiometer;- however alternatives are available from RS, Farnell and CPC (CPC RE53450K @ £4.35).
Also, Maplin no longer stock the digital panel meter (the Lascar DPM125). However this module is available direct from Lascar themselves (details below), from RS Components (who have trade counters that accept cash transactions) and from Farnell.
There is an alternative display module, the OEM33 made by Anders. These can be got from CPC (details below) who can deal with the public by mail order and from RS and Farnell - however the price the latter two charge is much higher than CPC so I can't see why anyone should buy from them.
IMPORTANT: The Anders module is slightly smaller but deeper, requiring a hole 41 x 22mm therefore a different layout within the box may be necessary to accommodate the extra depth.
I haven't yet used this module but there appears to be no reason for it not to work, however I can't guarantee that it will.
To use the OEM33 with this unit, the three jumpers J1, J2 & J3 must all be open and to prevent the extra annunciators operating it will be necessary to link pin "BP" to the following pins; BAT, C, F, ~, m, u, M, K, V, A and HOLD2. Link the decimal point pin "D3" to "XBP".

Supplier Part Stock No Cost Notes
Lascar DPM 125 DPM125 £22.98 Will sell direct
RS DPM 125 257-212 £22.98 From trade counters
Farnell DPM 125 178-650 £23.96
CPC Anders OEM33 PM11109 £14.57 Smaller panel hole & display. Deeper body

The following parts are taken from the Maplin and RS Catalogues, VAT should be added to all prices.
Component types aren't critical, you can substitute whatever you may have available although I'd recomend metal film resistors for their stability.

Qty Ref Item Description Maplin Ref. Maplin Cost Maplin Tot.      RS Ref. RS Cost
1 U1 OP177GP Amplifier NP16S 1.52 127-2868 2.26
2 Q1 2N7000 MOSFET UF89W 0.25 0.50 348-3824 0.09
2 D1,D2   Diode See note
1 P1 20K Multi-turn trimmer   See note 154-2252 1.15
1 P2 50K 10 turn potentiometer 164-4588 6.50
2 R4,R5 10K Resistor G10K 0.04 0.08 148-736** 0.03
1 R9 15K Resistor G15K 0.04 148-770** 0.03
1 R8 33K Resistor G33K 0.04 148-859** 0.03
4 # R1-R3,R6 100K Resistor G100K 0.04 0.16 148-972** 0.03
1 R7 10M Resistor M10M 0.21 135-667** 0.03
1 C1 22uF Capacitor WW72P 0.42 262-4917** 0.44
2 C2,C3 1uF Capacitor PY93B 0.21 0.42 262-5027** 0.23
1 MB2 Box LH21X 2.03 See note
1 Knob FK38R 0.42 581-868* 0.96
1 Switch RD20W 1.92 336-602 1.61
1 Battery Holder MJ45Y 1.95 593-704 2.02
1 PP3 Battery ND13P 2.54
1 3.5mm Jack Plug HF80B 0.42 449-994* 0.32
1 Cable gland UP95D 0.55 206-6080* 0.62
1 Photo-resist PCB BW19V £3.40 397-0097 3.92
(Enough for 4 - see note) Total £14.51
VAT £2.54
Total £17.05

# If the Maxtec sensor is used, one fewer 100K resistors is required and R6 should be increased to at least 1M - Maplin stock no M1M or RS no 149-228 (see the sensor specifications on the Notes on accuracy page).

* RS minimum order quantity of 5
** RS minimum order quantity of 10

If you choose to use a different type of MOSFET, be sure to use a "N"-channel type AND check the pinout matches that of the 2N7000, some alternatives will require either rotating or the leads bending.
D1 & D2 aren't absolutely necessary but if fitted, low leakage devices must be used. Unfortunately Maplin & CPC don't sell anything suitable. Farnell sell a FDH300 (367-989 @ 62p) & RS sell a JPAD50 (656-366 @ 90p) but I haven't tried this device. I'm still on the lookout for alternatives. General purpose diodes (1N4148 etc) WON'T do.

P1, 20K trimmer; Maplin don't sell a 20K side-adjust multi-turn trimmer, however they do a 50K top-adjust type (UH26D @ 84p + VAT). These seem to work satisfactorily, although the board must be partially slid out to adjust it.
Farnell and CPC can supply the correct part; Farnell 348-880, @ £1.01 and CPC RE00246 @72p

If you plan to use an etch-resist pen instead of the photo-artwork you won't need the photo-resist coated board - just plain copper clad board:

Copper clad pcb: HX01B @ £1.69 + VAT
Etch resist pen: HX02C @ £1.95 + VAT
Etching fluid: WF10L @ £6.80 + VAT or ferric chloride crystals: XX12N @ £6.37 + VAT

Unfortunately RS don't supply the MB2 box, however they do a quite extensive range of similar alternatives, it's up to you how much you're prepared to spend.

Also required is thin flexible (7 x 0.1mm) wire to connect things up and no more than about a metre of thin cable (screened) to connect to the sensor plug.

Maplin's on-line catalogue can be found at:

CPC's telephone no. is 01772 654455 and website:

RS on-line can be found at:

Lascar are at:

Lascar Electronics, Module House, Whiteparish, Salisbury, Wilts. SP5 2SJ. Tel: 01794 884567

Sub Aqua Products sampling adapter SN 5910:

Submarine Products
63-65 Twyford Road, Eastleigh, Hampshire, SO50 4ZG
Tel: 01703 612144

The Printed Circuit Board
Back to Analyser Page

Contact: Dave Cordes.