February 21, 2024

Since we created this tutorial, flashlight technology and parts have evolved at an extremely rapid pace. We try to keep linking to flashlights that will work for this build, but it’s hard to keep up. This means it’s important for you to understand the properties of the type of flashlight that will work for this project, so you can try out ones you can find yourself.

Really any light source can work for this type of projection, but LED flashlights are the cheapest, smallest, safest option as they are very bright, portable, and rechargeable, and don’t produce a lot of heat that could melt or ignite your stencil.

Brightness: In the build shown in our video we used a 1,000-lumen “tactical” flashlight we got on Amazon a few years ago for around $20. The brighter the better, but a 1,000-lumen flashlight with a fresh 18650 battery can throw a remarkably large and clear image in reasonable darkness. We have found essentially the same flashlight from a few different suppliers since then, and we suggest you look around yourself.

Batteries: Flashlights that use rechargeable lithium 18650 type batteries are a great option, because six or eight usable 18650 cells can be recovered from old laptop batteries, and a single 18650 can power one of these flashlights for 4–7 hours. (It’s OK if your flashlight uses AA or AAA batteries too, we just like lithium.) You’ll probably want extra batteries with you to last the night. Having a 18650 battery charger and extra batteries is a great investment. These are easily found online.

Reflector: It doesn’t matter so much which brand of flashlight you use, but we learned the hard way that some LED flashlights use a parabolic reflector to focus the light, and others use a clear plastic dome-like lens. Both work fine for a flashlight, but for use as a projector the ones with parabolic reflectors work way better, because they hold the stencil much further away from the hot LED and because they don’t complicate the optics.

Figure A

The ones with (good!) parabolic reflectors look like Figure A. Notice the shiny, concave internal surface with flat clear plastic cover. This is what you want.

You don’t want one that looks like Figure B. Notice the lack of shiny interior, and the convex, bubble-like clear plastic dome cover. Not great for projection.

Figure B

Size: The head of the flashlight we used fits perfectly into a 1½” plumbing fitting designed for ABS pipe, which made coupling it to the lens easy. Check the diameter of the head of your flashlight, and make sure it will fit into the smaller end of your plumbing coupler. The flashlight we currently suggest is the SupFire C8-T6 Tactical Flashlight, which comes with an extra 18650 battery and battery charger.

If your flashlight does not fit into your coupler, try wrapping several layers of fabric, tape, or other material around the flashlight until it fits loosely enough to move, but tightly enough to stay in place. We found that long, thin strips of velcro worked very well for this.