Some sensor and power questions

[gamefreak]

Alright Im working on a system to test the Temperature, level of water, and pH in a tank, the first 2 I have some parts for, the 2nd one is simple, but the third one is confusing me.

For temperature i have this: http://parallax.com/Store/Microcontrollers/BASICStampModules/tabid/134/txtSearch/temperature/List/1/ProductID/86/Default.aspx?SortField=ProductName%2cProductName

Checking its manufacturer it produces a current depending on the temperature, but I think the Atmgea168's analog pins require voltage, so I need a way to turn it into voltage, correct?

For the level of water I think I can put two wires into the tank and if there is a voltage the water is at that height or higher.

And for ph there doesnt seem to be that many options, i've seen some things about ISFET's twice, but not exactly sure on the operation and use of these.

Any suggestions on these?

Also, i want to have this hooked into an AC voltage with a DC battery backup, to get AC power I need to bring the voltage to about 3 volts higher then I need, rectify it then regulate it, but how can I have an almost instantaneous switch to the DC battery pack if the AC power were to go out?

[airman00]

For the temperature sensor :
Yes you need voltage for the analog pins , but since Voltage=current*resistance,  the voltage will change with current , but make sure to calculate a good value for the resistor so the voltage is below 5 volts.

For the Level of Water : The two wires idea is good , since water will short it out


As for the PH sensor:
http://www.trossenrobotics.com/store/p/4139-Ph-Sensor.aspx?feed=Froogle

http://www.vernier.com/probes/ph-bta.html

http://www.pondpetsusa.com/water_test_kits/pinpoint_ph.html
hack one of those

[gamefreak]

lol, I dont think my funders would like me spending 80 dollars on something that i will most likely fail at the hack process and break.

But while looking at the phidgets one and following some links, it did point back to ISFET's

Anyone know anything about these? Or have a better/easier way to get an accurate reading?

Alright, and looking at the temperature probes data sheet, it gives out 378 uA at 105 degrees celsius, 378 milliamps, so i need a 13 ohm resistor in order to have that just under 5 volts. But since I dont intend for the water to get anywhere near boiling, since I know nothing about fish, im going to assume that 90 degrees fahrenheit is a safe bet of the water is to hot. so 300 uA is a 16 ohm resistor.

[Admin]

pH in a tank . . . but the third one is confusing me.

You will need to find a pH sensor that outputs some form of electrical signal . . . hacking from there will be like your other sensors.

[paulstreats]

Swimming pools usually have ph monitors in them, but they usually need calibrating. I worked at a place a long time ago that had a pool and the ph sensor needed calibrating once a week.
What we did was measure the ph chemically and then set the digital unit to that measurement if it needed it. Quite often it wasnt necessary but they still require maintenance ove a couple of months. I dont think that electrical ph testing is reliable. Mind you im talking about over 10 years ago so there might have been some changes