Hi Robert

Regarding pH Sensor there are a few available i dont know from where :-) but
what you are looking at is meausuring conductivity between to probes -

Regarding CO2 sensor - thios is worse - I think you can by some some very
expensive chemical sensors how they work I dont know but there is another
solution to this problem.
CO2 has this strange property which makes it absorb infrared light at a
ceratin wavelength (I think 490 nm - check it out, I am really Really not
sure about this)  The solution then is to build a transmitter and a receiver
and measusure what Not coming through, This will give you an indication of
the CO2 amount in the air - a few problems however exists:

1. finding a LED which emits in that spectrum
2. Finding a Ligth sensitive diode which only responds in this spectrum - I
think a uk company manufatures diodes for this range Or You can get an
optical filter for the selected spectrum - you must make sure to get the
rigth componenets as you dont wnat to measure water vapor or whatever
happends to be between the LED and the SENSOR
3. LED driving circuit - normally the LED's of this kind must be pulsed in
order to get the maximal output - a suitable power transistor circuit must
be build
4. Temperature - These specialised Diodes are usually very temperature
sensitive so some kind of chopping might be needed- Here you can use the
fact that the diode must be pulsed anyway and make a sort of electronic
chopping :-) where you meassure the output from the receiver when the diode
is not running - this will give you a value dependent of the temperature -
puls the diode for a while and look at your input - with theese 2 values +
some calibration data etc you should know be able to calculate the CO2
amount in the air with a good precision - at the same time you avoid to
build ekstra circuits for temperature compensation.
5. CO2 is - what many people dont know - very dangerous - and a
concentration of just 10% will cause unconsciousness - remember this when
you calibrate your circuit :-) It will make your plants grow - but be maybe
not you  :-)


The above solution should work - I did it once many years ago as a project
during my studies
I believe with proper chosen componenets (PIC etc ) you should be able to
keep the cost down to less than say 150$ fro the total circuit - There are
comercial available products doing exactly this - but they are priced much
much higher

feel free to share all your future earnings with me :-)

regards

Peter Neubert
Mauritius


>Howdy,
>
>
>
>        I am interested in finding some information for a PIC project.
>
>        Is there a sensor that can measure pH that is cheapish and can
>        say be connected to a PIC (similar to a DS1820 temp chip)?
>
>        Also, is there a sensor which can measure the amount of CO2 gas
>        in a given area?? again, it would be good to connect this to a PIC
>        chip.
>
>        The reason is that I am interested in starting a little hydroponics
>        set up, and any information on these (including the hydroponics
:-))
>        would be of great help.
>
>
>Regards,
>
>
>Rob Walker.
>