The　mobile　industrial　human-machine　interaction　plays　an　important　role　in　the　industrial　internet　of　things,　since　the　engineers　can　use　a　mobile　device　to　interact　with　machines　that　greatly　improves　the　efficiency　and　safety.　Nevertheless,　connecting　to　a　specific　machine　becomes　a　non-trivial　problem　due　to　the　massive　machines　in　the　network　that　make　the　connection　list　too　long　to　identify　the　target　machine.　Some　solutions　such　as　QR　code　scanning　and　proximity　estimation　have　been　proposed　to　solve　this　problem.　However,　they　have　limited　performance　in　scalability　and　accuracy　correspondingly,　and　thus　cannot　satisfy　the　requirements　in　most　applications.　Observing　the　fact　that　the　engineers　generally　interact　with　the　machines　in　their　line-of-sight,　we　propose　the　LightCon　scheme　which　adopts　proximity　estimation　to　estimate　the　machines　in　the　line-of-sight,　and　controls　the　display　module　of　machines　to　show　different　visible　symbols　(colors　or　numbers).　To　connect　with　a　specific　machine,　the　engineers　just　need　to　select　the　corresponding　symbol　on　the　mobile　device.　Therefore,　they　do　not　have　to　remember　the　trivial　address　of　each　machine.　Furthermore,　the　symbol　assignment　algorithm　is　designed　to　reduce　the　complexity　of　manual　symbol　selection,　and　its　performance　is　analyzed　theoretically.　The　performance　of　LightCon　is　evaluated　in　the　testbed,　and　the　experimental　results　prove　that　LightCon　is　a　promising　solution　to　simplify　line-of-sight　connections　with　low　complexity.