Not necessarily arduino, but I'm working on building a lightsaber modified from one of the toy electronics. The toy uses 3 AAA batteries, and the board says 5 V. I was wondering if there's a way to replace these batteries with something smaller. Like coin batteries or something? I know I can make the voltage add up, or use a transistor I think, but what's the amp situation I'm looking at? Is it possible?
Three 1.5 volt button cells such as A76 could work but not as long as AAA batteries. I cannot say how long until you select button cells and until the current consumption of the light saber is determined.
What "board says 5 V"?
An Arduino will run very nicely from 3 alkaline AAA batteries.
If you want nice flashing lights, the three alkaline AAA would be the very minimum you should use. I keep an eye out for discarded "lightsabres" after the New Years Eve fair celebrations. These use (three) button batteries (not coin cells which are used for devices operating intermittently over many months, such as scales) and give a nice display for maybe an hour in the dark at night.
A cellphone external booster battery is a good power source for 5v electronics, and they come in a wide variety of sizes. (Though “smaller tha 3xAAA is probably pretty uncommon.