DIY ph amp

DIY · Posted: Fri May 18, 2007 5:30 pm Post subject: DIY ph amp

I've been meaning to make a pH amp for a while now, Bad Inferno's 1-wire monitoring system and Bald_noggin's pic based controller gave me the spark to continue on with what I'd already started.

I wanted a pH amp that could be connected to different devices, for example a 1-wire ADC or a PIC. To be the most portable I wanted the following things

. Operate from a single supply, preferably 5v
. Have a gain of around 8.2 to provide a resolution of .01 PH using 10bit adc found on most PIC's @ 5v.
. Calibration adjustment pot's included to reduce need for memory usage on a PIC due to needing a programmed calibration routine.
. Work with 1-wire ds2450 as well as PIC, although ds2450 accuracy is only 8bit, it can be configured to read at 10.
. Simple & cheap to build, the hardest part should be making the board

So on to the circuit, I'm no electronic's guru but I do like to DIY... I've looked at various pH circuits on the net and in project books and decided on the basic example design given in the LM6001 opamp datasheet and added various features found in other circuits.

The spec is a usable output of just above 0v to just below 5v and the intended 0.01 resolution on a 10bit ADC. The readable range is 2.0 - 12.0 pH.. Since I'm not planning on running an aquarium lower than a pH of 2.0 or higher than 12.0 it's fine for me.

The first pic is the circuit in operation on a breadboard, it is connected to my other stalled project - a pic based controller...
I'm really happy with the operation so far so the next step is to make a PCB and house it in a shielded enclosure.

Second pic is the actual circuit, some values may still need some "tweaking" otherwise it seems to work ok.

More to follow...

(Info added, circuit updated 19/5)

Bad Inferno · Posted: Fri May 18, 2007 9:15 pm Post subject:

Great job...Tell me about the 7660. I think what I am seeing is a input of 5VDC and a +/- output of what voltage....?

I built a cct for a pressure sensor had all sorts of trouble ranging the thing. its output was 4-20ma and I put it across a 250 ohm resistor to give me 1-5VDC....however I could not get the span I wanted and have ended up with a span of 1.5 volts on my 1wire A/D input which can take 0-5VDC. I don't want to steal your ph amp thread so I might PM you my cct. If you wouldn't mind having a look. I use this pressure sensor as level monitoring in my reservoir. The sensors provide something like 0-10mv for 0-10 kpa. I assume I can use your exact cct just change the gain ?

http://www.freescale.com/files/sensors/doc/data_sheet/MPX2010.pdf

rob

DIY · Posted: Fri May 18, 2007 9:29 pm Post subject:

I actually was considering the same thing after I finish some of the things I have started... I even did a quick breadboarded circuit some time ago but didn't get it quite right at the time..

Very similar circuit to the one above and yes different gain, I think I have a couple of examples somewhere I used when I tried it. I'll do a hunt and see what I have. pm me your email and I'll send you what I have, and happy to look at what you have so far.

The 7660 is an inverter so for 5v in gives -5v out, it has limitations like it can't provide much current, and even at a couple of mA drain will start to drop voltage - eg +5v gives -4.7v but for an opamp or 2 it's handy.

DiscusMad · Posted: Sat May 19, 2007 12:06 am Post subject:

I wish I had great knowlage on electronics
the best I ever did was some sort of keyboard at high school

DIY · Posted: Sat May 19, 2007 12:15 am Post subject:

DIY · Posted: Sat May 19, 2007 2:19 am Post subject:

Update:

I've spent this evening creating a pcb board layout in cadsoft's eagle (it's free!), the main criteria was it should fit inside the 100x50x25mm diecast enclosure I got today.

Pic's are of the actual board layout and a printed copy of the layout sitting near the enclosure after i checked the dimensions.

DIY · Posted: Mon Jul 16, 2007 12:02 am Post subject:

Been meaning to update this thread for a while Embarassed

I'll let the pics do the talking Laughing

DIY · Posted: Mon Jul 16, 2007 12:04 am Post subject:

Works great connected to my breadboarded PIC controller. I was able to calibrate without any problems, although in the attached pic the pobe is just in storage solution. The RJ45 connection is based on the 1-wire standard for analogue sensors.

DIY · Posted: Mon Jul 16, 2007 12:14 am Post subject:

And finally a pic of it connected to a DS2450 1-wire ADC which is in turn hooked up to a NSLU2 (not pictured) as described in the aquarium monitor - controller thread. I've also copy & pasted a pic of the raw voltage (volt.A) from the owfs web server running on the NSLU2.

If anyone ever builds one to use with a DS2450 like this, the basic calculation is (Raw Voltage/0.48859)+1.88 = pH units. So for the raw voltage in the pic of 2.11386 that equals a pH of 6.20 (rounded to 2 decimal places)

I'm happy with the result ThumbsUp

Bad Inferno · Posted: Sun Aug 12, 2007 6:15 pm Post subject:

Have you seen the cheap pH meters on ebay ? take a look they seem OK....I was thinking if you could get to the back of the seven segment displays we maybe able to read off the value displayed as digital inputs and decode the value in a PIC ?

DIY · Posted: Sun Aug 12, 2007 7:22 pm Post subject:

Anything's possible Wink

To decode the seven segment display might use up a lot of I/O pins though. There's a lot of different cheap meters on ebay atm not just Ph so it is tempting to try a couple of them.

Bad Inferno · Posted: Sun Aug 12, 2007 7:36 pm Post subject:

i was thinking you could multiplex each of the 7 segment displays and just use external cmos ? switches to select either digit 1 / 2 or 3...as my pH provides a 0-5VDC output I'm OK however of others this could be possible without having to really build too much. I just got one of those "black" pH probes off ebay ($15.00) ..it took a week in the tank and it has settled in now. Don't really like the fact I cannot see the reference KCL solution in the probe. I still think I like the "green" tube style probes selling on ebay. I also found that the fluval pipe fitting with the two suction cups (it looks like a plastic device that the heater normally get attached with) is just the correct size to fit the probe into. So you attach this inside the tank and the probe just slips.

ILLUSN · Posted: Sun Aug 12, 2007 7:37 pm Post subject:

careful with the cheap meters on ebay, i had an orp meter that said my redox varied from 260-1999mv over a 2 day span (it also said the tap water was -160mv on day 3 Wink

C4Vette · Posted: Sun Sep 09, 2007 6:25 am Post subject:

DIY · Posted: Sun Sep 09, 2007 9:59 am Post subject:

Bad Inferno · Posted: Sun Sep 09, 2007 11:06 am Post subject:

Welcome C4Vette to the world of Discus !

Cheers rob

Bald_noggin · Posted: Sun Sep 09, 2007 11:40 am Post subject:

Hmmmm, C4Vette something I like about that name! Welcome!

C4Vette · Posted: Mon Sep 10, 2007 12:40 am Post subject:

DIY · Posted: Mon Sep 10, 2007 8:51 pm Post subject:

C4Vette · Posted: Mon Oct 08, 2007 4:39 am Post subject: