I use a transistor driven by ttl or cmos logic. Most pen lasers use 3 batteries at 1.5 volrs each so using 5volts (ttl logic voltage) will damage laser. I usually put a couple of diodes in series with the transistor and the laser.
+5 volts---diode---diode---+-laser---collector of npn transistor. common ground---emitter of npn transistor. ttl or cmos or 555 timer------1K resistor-------base of npn transistorDave