Laser

I plan to use a key chain laser for the controller. I bought it for 1$ from eBay if I remember correctly. It works with a single 1.5V battery. I will push its switch button on and fix it with tape or shrink tube. Then I will remove the battery and solder some wires to the contacts. The wires will be connected to the Arduino via this circuit. The laser is probably a simple laser diode and some resistor to control the current. If it were so, then the voltage does not really matter. What matters is the current, so we can drive the laser from 5V with some additional current limiter resistor. Here is a small experiment to find the required resistance:

laser

The Arduino can source 40mA current. That means minimum 125 ohm resistance at 5V. So if we use a a potentiometer bigger than that we are on the safe side from the Arduino’s point of view. I would also recommend to use a separate power supply first instead of the Arduino. I assume the laser requires about 20mA so I will use a 1K potentiometer that can limit the current to 5mA. I will set the potentiometer at the maximum resistance first, then I decrease it until the laser lights up properly. Then I check the current. If that is OK for the Arduino, I simply replace the potentiometer with a corresponding resistance.