Fits 3 (three) 9 Volt Batteries, wired in parallel to power an Arduino UNO R3. This case includes 2 different lids. One standard top and one that will hold an Arduino UNO R3. The case itself can be stand alone or mounted. The (4) holes fit the Lego round shafts in technic kits. If you don't have a 3D printer but you're interested in getting one of these we can print you one at www.badbrick.com/catalog It will come fully wired up and ready for 9v batteries.In MakerWare, I modify the "Medium" settings: Quality: Layer Height = 0.2 and I increase the temp to 240C to let the little larger prints fuse better. All other defaults are the same.