Hi,
I want to bend a cooper wire (length= ~1m) several times in a loop in order to make a rudimentary coil. Having one side of the wire connected to 5v and the other to the ground, I want to know if it's safe to give power to the Arduino board in a situation like this or it's necessary to insert a resistor and if so, what value of the resistor is recommended?
Thanks in advance!
You need to know the thickness of the wire and thus its resistance-per-unit length. For copper of cross-sectional area of 1 square mm the resistance per meter is about 17 milli-ohms (5V would push about 300 amps through the wire melting it very quickly!)
MarkT:
You need to know the thickness of the wire and thus its resistance-per-unit length. For copper of cross-sectional area of 1 square mm the resistance per meter is about 17 milli-ohms (5V would push about 300 amps through the wire melting it very quickly!)
More likely melting the voltage regulator 
If I understand what your asking, you want to use a Arduino to measure the resistance of a homemade coil. If that is the case, you would want to use a constant current circuit (A LM317 reg can provide such) and then use the Arduino analog to digital converter to measure the voltage across the coil. Since the resistance is low, the current must be fairly low as well to keep the voltage below the 5V max for the Arduino...
No, sorry but I forgot to specify what I am using this for.
I want to make a pseudo-NFC transmitting device (google poor man's NFC). It essentially uses a magnetic field to transmit data one-way to a device with magnetometer (an Android phone). The coil will be used to create the magnetic field and also will be switched on and off pretty fast (once every 90-100 ms: ~10bps). This experiment is for educational purposes only and I don't want to accidentally fry the Arduino board because of a small experiment with no purpose in real life.
Thanks for the answers provided so far!
razboss:
No, sorry but I forgot to specify what I am using this for.
I want to make a pseudo-NFC transmitting device (google poor man's NFC). It essentially uses a magnetic field to transmit data one-way to a device with magnetometer (an Android phone). The coil will be used to create the magnetic field and also will be switched on and off pretty fast (once every 90-100 ms: ~10bps). This experiment is for educational purposes only and I don't want to accidentally fry the Arduino board because of a small experiment with no purpose in real life.
Thanks for the answers provided so far!
Not sure what NFC stands for (near field communications?) but the the design of the coil will not be based just on it's DC resistance, but also on it's induction value which will determine it's impedance at your choice of frequency of operation. So you are therefore working in the world of AC theory not just basic DC theory. As such you are really just describing an RF information link. An RF signal, once propagated from the circuit, contains both a E field wave (electrical) and a H field wave (magnetic) that encompassed what a electromagnetic wave is composed of.
So I suggest you study and research simple RF communications methods and circuits. You will need to know more specific information about the magnetometer sensor on the Android, such as what is it's frequency response range. A raw CW (Morse code) on/off type signal rate of 10bps would result in a 10Hz carrier frequency which would require a huge induction value for the coil usually requiring a iron core, much like 50/60Hz rated power transformers, which might be impracticable for you project, depending on specific power level desired for whatever your specific maximum 'near field' distance requirement is, which is then dependent on the sensitivity value of the magnetometer. Better to utilize a much higher 'carrier frequency' (but still detectable by the magnetometer) which you can key on and off at whatever slow information rate you wish. Again focus on the specifications and capabilities of the magnetometer first, it's frequency range, sensitivity, etc, that will dictate what is possible and practicable.
Lefty
Thanks a lot for the answer, I will try and follow your advice and update the topic as soon as I will get some infos about the magnetometer.
The answer to the question, "Is it safe to connect a meter of wire between a digital output and ground?" is, "No." The resistance of a meter of #30 AWG wire, roughly equivalent to 0.05 mm2 - some really skinny wire - is about 1/3 ohm. That'll carry about 15A at 5V. From the ATmega328P datasheet, the absolute maximum rating for DC output current from a pin is 40 mA, which corresponds to a resistance of 125 ohms. If you connect the coil directly to the output, the pin won't actually delivere 15A; it'll deliver as much as it possibly can, and that's too much for it to deliver safely in the long term. It may not blow up in your face, but it'll never be quite the same, either.
Googling "poor man's NFC" yields a lot of hits, but they all seem to eventually lead here: http://jdesbonnet.blogspot.com/2011/05/arduino-to-android-io-on-cheap-aka-poor.html. The schematic shows a 120 ohm resistor in series with 30 turns of wire in a coil with diameter=1 cm, connected between pin 13 and GND, with a flyback diode across the coil. The photo, though, omits the resistor and diode, so the author's arduino is driving what amounts to a dead short circuit. He apparently thought that pin 13 already had a resistor in series for current-limiting for the LED, but the resistor is really in parallel with the pin. He says, "no harm done," which presumably means that his Arduino hasn't failed quite yet.
The code shows a bit rate of about 7 bps, limited by the Android's sample rate for its internal magnetometer. Bit transmission is coil on=1, coil off=0. The magnetometer's purpose appears to be for magnetic compass applications, so it's sensitive to DC fields. With this bit rate, I don't think that the system's RF characteristics are particularly important.
There's some code for the Android, to persuade it to decode magnetometer readings into serial data.
With the resistor installed on pin 13, the output current is still too high: 5V/120 ohms is close to 42 mA, and the LED draws nearly 7 mA, for a total of around 48 mA - still more than the absolute maximum. If you want to do this project, you'll want to beef up the resistor a bit, and drive the coil from a different pin. That may not develop enough magnetic field for reliable detection in the Android. If that's so, you'll want to buffer the output with a transistor, and you'll still want the flyback diode.