The suspect of an armed robbery in Brighton is behind bars after the man he robbed helped officers ping his stolen cellphone to the suspect’s location.Boston police said they were notified of an armed robbery in the area of Strathmore Road in Brighton Friday night and were told that a man had been robbed at knifepoint of his gray Adidas backpack containing a cellphone, personal papers, a wallet, passport and over $400 in cash.Police said the man assisted officers in “pinging” his cellphone to a home in Brookline that was known to police.