ines ▁invisible ▁light ▁onto ▁sens ors ; ▁if ▁someone ▁or ▁something ▁blocks ▁the ▁beam ▁the ▁door ▁is ▁triggered ▁open . ▁ ▁A ▁motion ▁sensor ▁which ▁uses ▁low - power ▁mic row ave ▁rad ar ▁for ▁the ▁same ▁effect . ▁ ▁A ▁remote ▁sensor ▁( e . g . ▁based ▁on ▁inf ra red ▁or ▁radio ▁waves ) ▁can ▁be ▁triggered ▁by ▁a ▁port able ▁remote ▁control , ▁or ▁is ▁installed ▁inside ▁a ▁vehicle . ▁These ▁are ▁popular ▁for ▁gar age ▁doors . ▁ ▁A ▁switch ▁is ▁operated ▁manually , ▁perhaps ▁after ▁security ▁checks . ▁This ▁can ▁be ▁a ▁push ▁button ▁switch ▁or ▁a ▁sw ipe ▁card . ▁ ▁The ▁act ▁of ▁pushing ▁or ▁pulling ▁the ▁door ▁triggers ▁the ▁open ▁and ▁close ▁cycle . ▁These ▁are ▁also ▁known ▁as ▁power - ass isted ▁doors . ▁ ▁In ▁addition ▁to ▁activation ▁sens ors ▁automatically ▁opening ▁doors ▁are ▁generally ▁fitted ▁with ▁safety ▁sens ors . ▁These ▁are ▁usually ▁an ▁inf ra red ▁cur tain ▁or ▁beam , ▁but ▁can ▁be ▁a ▁pressure ▁mat ▁fitted ▁on ▁the ▁swing ▁side ▁of ▁the ▁door . ▁The ▁purpose ▁of ▁the ▁safety ▁sensor ▁is ▁to ▁prevent ▁the ▁door ▁from ▁coll iding ▁with ▁an ▁object ▁in ▁its ▁path ▁by ▁stopping ▁or ▁slow ing ▁its ▁motion . A ▁mechanism ▁is ▁set ▁in ▁modern ▁automatic ▁doors ▁to ▁ensure ▁that ▁door ▁will ▁be ▁in ▁open ▁state ▁in ▁case ▁of ▁power ▁failure . ▁ ▁O thers ▁▁ ▁Up - and - over ▁or ▁overhead ▁doors ▁are ▁often ▁used ▁in ▁gar ages . ▁Instead ▁of ▁h ing