This tiny coin cell battery holder is ideal for small portable or wearable projects. It holds one 20mm coin cell (CR2032 is the most popular size) to generate 3V nominal. 20mm coin cells are popular as they are used in keychain lights and remotes and other small topys, so they are easy to get and fairly low cost. And speaking of lights, these battery holder includes three "LED" modes: quick flashing, slower flickering, and static on. Press the button to cycle through the modes. Although designed for LED jewelry, it could be used for other projects that need simple cycling.