DIY Oxygen Analyser Parts List

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).

Note
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:

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.