Ardiuno fish tank heater

I develop photographic film and I want to use a thermoelectric element to maintain a tub of water at either 20C or 38C, (±.5C) depending on my process.

I know there is a PID library but I doubt I even need that; I could do what I want to do in software pretty easily.

Two issues I have are thermometry and price.

The last arduino board I bought was the StickDuino, which was about $20 w/chip. Not bad at all considering the capabilities, but this is a pretty simple app. What’s the cheapest arduino board out there?

Then there’s thermometry. Sure thermocouples exist but you need another junction to use them. Since I don’t really need linearity, only two points, I could really use anything that varies resistance with temperature; such as a coil of wire or a reversed-biased diode or anything like that. Any ideas as to a cheap temperature transducer would be appreciated.

Well, afraid I’m no help in Thermometry.
Though, if you’re looking for the cheapest Arduino, you can purchase the Atmega8, 168(etc), flash them on your board, then just pop them out, solder it up / hook up to breadboard and you’re good to go!

For a standalone Arduino, you’re going to need to either set the internal clock(I believe this requires some code change), or get an external 16 MHZ crystal.
http://itp.nyu.edu/physcomp/Tutorials/ArduinoBreadboard

Also, here is one of my favorite Arduino help-sites.http://freeduino.orgAlmost everything you’d ever need! :stuck_out_tongue:

Price wise, standalone Arduino is your best bet, as you can get Atmega8 for $2-3 each on Ebay. You will need to do a search on putting the Arduino boot loader. (Or any other you’re comfortable with)

Also, take a look here, Todbot post using just an Atmega8 and very few components for a standalone Arduino!
http://todbot.com/blog/2009/05/26/minimal-arduino-with-atmega8/

Hope this helps.

I actually already have an arduino-flashed ATMega chip. I could plop it in one of my micro-breadboards, but I think that besides 5v DC, I need a crystal and a cap or an oscillator or something else. Maybe I’ll find that info on one of those sites.

As for the temperature sensor, it seems they’ve already invented a custom solution for my application. I believe they are calling them “thermistors”. Amazing what you can buy off the shelf these days!~

I’m working on something similar - specifically a home brewing immersion heater controller.

I’m using a Duemilanove for the development but I’ve put a socket for a ATmega168 on the PCB with an oscillator attached. The idea being that once I’ve developed the software, I drop £4 of micro in and not £25 of Arduino.

On the socket, I’ve got a 16MHz crystal and two 100nF caps on the supply lines. Everything else is connected to the pins that the Duemilanove plugs into as per http://www.arduino.cc/en/Hacking/PinMapping168

re: temperature control how are you proposing to sense the temperature? I’m sensing the temperature of the wall of my metal heater tube and that is about 3ºC higher than the average temperature of the brew I’m heating.

Plus, at low temperatures, convection currents in water are reeeeeaaallly sloooow. Some kind of agitation / mixing will achieve a much more even temperature and simplify your sensing requirements