Northeastern police Detective Jonathan Boyd was directing traffic during student move-in when a motorist flagged him down. It was a moment that saved a Boston resident’s life.

That afternoon of Sept. 5, the driver pointed Boyd to a 68-year-old man on St. Stephen Street who was having trouble breathing.

“He kept saying ‘I’m fine, I’m fine,’ but he was holding his chest,” Boyd recalled. “I asked him if he had a heart condition, and he pulled up his polo shirt to show me the scars from heart surgery.”

Suddenly the man fell, and Boyd — a longtime CPR instructor at Northeastern —launched into action. He called for help and began administering CPR to the stricken man.

“I didn’t have time to be nervous,” said Boyd, who joined the Northeastern department in 2004, after a year on the Bristol Community College police force. “I just went into my training mode.”

Colleagues later told him a crowd formed to watch, but “I never even looked up,” Boyd recalled. Working feverishly, he revived the victim with the help of a quickly responding Boston Fire Department crew, and awaited an ambulance.

When the ambulance arrived the man again slumped into unconsciousness, but Boyd, EMTs and firefighters managed to revive him yet again. He was taken to Boston Medical Center where, Boyd learned recently, he is expected to recover fully.

Northeastern public safety leadership presented Boyd with a letter of commendation at an assembly of his colleagues the next day. “You have prepared for this type of situation for years, and handled it with the skill and composure of a true professional,” James Ferrier, associate director of public safety, told the detective. “Your performance is a credit to all of us in the division.”

President Joseph Aoun thanked the officer for his quick and decisive action. “This institution has great values, and you embody them,” he told Boyd. “Thank you. You make us proud.”

Boyd, who holds a bachelor’s from Northeastern and is studying for a master’s in criminal justice leadership, was promoted to detective last year. In 2006 he was honored by the Massachusetts Association of Campus Law Enforcement Administrators for his outstanding community policing initiatives on campus.