This is sized to fit a uxCell AA battery case and an HM-10 Bluetooth module - https://smile.amazon.com/gp/product/B00E0IXRUS/ref=oh_aui_detailpage_o02_s01?ie=UTF8&psc=1 * Note: This isn't a genuine HM-10 module - it has the bolutek firmware. https://smile.amazon.com/gp/product/B01GPGI3FQ/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1