repeating_command_block, set to Unconditional + Needs Redstone, with the following command: execute @a[x=0,y=20,z=-1500,r=50,tag=microbattles] ~~~ testfor @s

After powering the command block with a redstone_block and then destroying it, the comparator signal continues to be powered (tested and connected with a repeating /say command_block).