This is beginner kind of a question. I haven't studied electronics nor engineering. So please, don't blame.
I got arduino compatible mega 2650 board (picked up at local store on sale). And ordered few temperature sensors. Also figures out that house thermostat has tr/rx pins for communication.
Idea is to place two temperature sensors in each room and every minute or some collect temperature.
With some programming skills I can make a visual 3d model of house and see temperatures in each room.
Next make arduino communicate with thermostat to automatically control temperatures in each room separately.
But since I am new into electronics, I have no idea how long cable from arduino to temperature sensor can be. 1-3ft? How can I make it at least 10 feet?
I don't think you should have an issue with 10 feet. best thing to do is get an arduino and one sensor and do some testing to see how your idea works in practice.
I would be very interested in how you would control the temp of individual rooms that sharw a thermostat. maybe servo controlled air baffles?
If they are active ones that send a coded signal using the 1-wire protocol or similar shared bus (e.g. DS18B20), then the cable capacitance may affect transmission, so more than a few metres of cable may be problematic. If they are thermistors or DC sensors such as LM34 and LM35, then you can get away with long cables if you follow the recommendations on the datasheet; but if the cable is too thin then its resistance may cause errors in the temperature reading.
I suspect you'll be OK with 10 feet or more. What tempertaure sensor are you using?
In general, there are two potential issues with long cable runs. Capacitance and noise pick-up. Capacitance affects high frequencies. (This is not an issue since the temperature and related voltage changes slowly.) In cases where you are dealing with large amounts of current or low-impedance loads (which you are not) the resistance of the cable (which is related to cable length and wire gauge) can create a voltage drop.
Noise is a potential issue for you. The biggest source of noise is the 50/60Hz electromagnetic field from your house wiring. If this is a problem, you'll get unstable temperature readings. Noise pick-up can be minimized by using [u]shielded wire[/u] (for the same reason, shielded wire is used for audio cables). And depending on what's "allowed" by the temperature sensor specs, a resistor and/or a capacitor across the Arduino input will help to kill noise.
To use long cable lengths with the DS18B20, try a 1K pullup resistor in place of the usual 4k7. See Mixed-signal and digital signal processing ICs | Analog Devices for guidance on cabling. That document suggests that you may be able to achieve a total cable length of 200m. However, one user is reporting a problem with just 10m of cable in another thread on this forum.
