I was never sure if Batgirl had a Batsignal, but it doesn't really matter. I'm gonna build my own using Arduino, some LEDs, and these instructions anyway. And then I'll track down a slinky costume and take down some troublemakers.

You can find all the details for Batsignal projector (including stencil patterns) at Thingiverse. Fun part is that you can even switch out the stencils just in case you decide that you want a different superhero identity. Spacegirl-signal, anyone? [Thingiverse via Make]