Minecraft wiki entry for Redstone_components, associated text: A redstone comparator is turned ON by a power source at its input or a power source separated by one opaque block from its input. Power sources include any powered component, a non-empty container, a container minecart on a detector rail, a command block that has run its last command successfully, a cauldron containing water, an end portal frame with an eye of ender, or a jukebox with a record. ) either at its back or separated from its back by an opaque block. It is not affected by blocks beneath it or above it, but its signal strength can be modified by signals from its sides (see below).