Use a thin cobber wire so that you get enough resistance. Or make it physically big so that it doesn't get hot (I'm not saying that it won't drain your batteries).
Unless you're using PWM to control the magnet, a current limiter circuit will drain the batteries fast and the effiency will be poor.
No point with 4xAA's - limiting the current would simply make the magnet weaker. As suggested, wind the coil using suitably thin wire to give the correct result you want, this will give maximum efficiency, but it will run hot.
Will it makes the battery weaker if the electromagnet switched on for couple of minutes ?....the coin need to be lift up from one point to the other....plus there are some circuits associated in the robot and 5V DC battery is the only main supply.
Will it makes the battery weaker if the electromagnet switched on for couple of minutes ?....the coin need to be lift up from one point to the other....plus there are some circuits associated in the robot and 5V DC battery is the only main supply.
Again it depends how strong you want the magnet - winding it with lot's more turns of thinner wire will reduce power consumption, but will make it weaker. I suggest you try various different windings to find what works best.