Powering a laser pen
Similar to a flashlight, one battery contact is the outer metal case and the other contact is the center. A laser diode will be sensitive to polarity. If you still remember which way the batteries were inserted before the teardown, the tab at the end of the PCB is the inner contact for the center battery terminal (most likely positive I'd guess).



You do promise to be careful not to burn your eyes out with what is presumably a Class I laser device, right? Wikipedia Laser Saftey is required reading.