I was powering it with a little more than just enough to light it @ 2.1v, hardly enough to form any noticeable unidirectional emission. A little more power, and a huge contrast forms between light going in many directions and the main beam. this one was supposed to run on 4.5v of button batteries. @4.8v and point blank range shining on a black sharpie spot on my skin, no lens in the way, I felt no heat at all, but maybe I could if it was focused at that distance (about 1mm from the diode). Just putting the <5mW red laser power in perspective.
My only problem now is finding the right size pen or junk laying around to make a pen host.
Are you planning on using this little red diode you found, Nutball? If you do, the Vf is much lower than 4.5 volts, so you will need a driver for it. The Vf is probably somewhere around 2 volts. But, don't think you can power it with 2 volts in batteries without a driver because that could still burn the diode out.
It is complete with a switch, battery spring, and resistor for limiting current. So I'll just give it 3x 1.5v batteries like what it took, but I'll use batteries bigger than buttons, but not bigger than AAA
Is there only a resistor to limit the current? That is incredibly cheap, if that is the case. Since laser diodes effective resistance decreases with heat and current, it will only limit it until it no longer can. But, if the diode is rated for much more current than what is allowed to flow through the resistor, it could work and not fail.