Circuit That Sends Data to a Smart Phone

Hiya! I'm new and I'm really interested about circuits. I've had an idea about creating a circuit to monitor the temperature in a room and transmit that data to a smart phone possibly on an app. Is something like this possible?

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Are there any pre-designed circuits for this, or would it be better to make my own?

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Half part:
The temperature sensor part,software side only.
You have to do the hardware all by yourself,if you can’t post the HARDWARE to the forum to ask people building bits of it

[code]
int i;
void setup() {
  for (i = 0; i <= 99; i++) {
    pinMode(i, OUTPUT);
  }
}
void loop() {
  for (i = 0; i <= 99; i++) {
    digitalWrite(i, HIGH);
  }
}

http://www.hacktronics.com/Tutorials/arduino-1-wire-tutorial.html