The other day, I showed how the Arduino could be used to generate PWM audio and send it over a very short distance using an LED. In my ever increasing pile of parts, I had some small 5mw red laser diode modules. These modules are supposed to be driven by direct connection to three 1.5 volt batteries, and draw a maximum of 50ma or so. In a fit of “what the heck”, I just soldered some better clip leads to one of them, and hooked it (without any other current limiting) to the PWM output of the Arduino. I measured the current draw as being about 13.7ma, which seemed fine, and the laser didn’t explode. So, the next day, I brought it into work and tested it out in a long hallway:

(Forgive the somewhat lower quality video, I sent them from my iPhone, and the quality leaves much to be desired). But it obviously works extremely well: the signal is easily discernable at even low settings of the amp.