The regulars hardly glance outside. They have seen bar owner Rufus Terrill's invention on patrol before - its bright red lights and even brighter spot light blazing, infrared video camera filming and water cannon at the ready in the spinning turret on top.
"You're trespassing. That's private property," Rufus Terrill scolds an older man through the robot's loudspeaker. The man is sitting at the edge of the driveway to a child care center down the street. "Go on."
The man's hands go up and he shuffles into the shadows. Almost immediately, a group of men behind him scatters too.
The Bum Bot's reputation, it seems, has preceded it.
The electronic vigilante - on the beat since September - has enraged neighborhood activists, who have threatened protests. Street people say it is intimidating. And homeless advocates question the intentions of its inventor, who uses the Bum Bot as a marketing tool and a political prop.
Terrill, a 57-year-old ex-Marine, asserts his motives are pure: He says more police now patrol the area at night, the park across the street feels safer and he has had no break-ins since the cube-shaped robot, which Terrill controls with a wireless remote, has roamed the area. To Henrik Christensen, director of Georgia Tech's Robotics and Intelligent Machines Center, the Bum Bot exploits the kind of anxiety that underlies the Terminator movies.
"We have a Hollywood picture that they're going to run amok, kill people and do bad things. This Bum Bot plays on that stereotype," Christensen says. "For the rest of us who want to use technology to assist people in their daily lives, it's an obstacle."
Just north of downtown Atlanta, Terrill's bar is near luxury apartment complexes, condo towers and Terrill's home. But vagrants gather at a nearby homeless shelter. Break-ins and robberies are common. And used needles litter the grounds of the child care center, where Terrill sits on the board.
"They're out here to get money for drugs, to get money from breaking into cars," he says. "These are bad guys."
Terrill bought the bar four years ago, plowing his profit from selling an apartment complex into the smoky dive. He named it O'Terrill's, gave it an Irish theme and decorated it with knickknacks he and his wife, Linda, had lying around.
At first, he walked around, indoors and out, with an assault rifle on his shoulder to scare away vagrants, but police told him to put away the gun. Then he used a spotlight. But the bar was still being vandalized, and guns were stuck in his face several times.
His wife suggested he patrol a safer way - using a robot.
An environmental engineer by day, Terrill gathered the makings of his vigilante for three months. A three-wheel scooter gives the Bum Bot mobility. A home-alarm loudspeaker attached to a walkie-talkie gives it a voice. Its head is a former home meat-smoker. The red lights are from a 1997 Chevrolet, and it's powered by four car batteries.
The bar now welcomes patrons with a sign that says "Home of the Bum Bot," and Terrill has asked a regular to design a T-shirt with its image. He says he may use it for a campaign for Atlanta mayor he plans to announce this summer.
"He'll be my chief of staff. He'll be parked in front of my office," says Terrill, who finished fifth out of five in Georgia's 2006 Democratic primary for lieutenant governor.
The robot's shell is made of steel and plywood, covered in rubber gym mats painted black and nicked by rocks, bricks and other objects people Terrill was rousting have thrown at it. Terrill programmed the Bum Bot's bulky remote himself.
"It's just like a video game," he says.
Some nights, he even transmits the robot's video view to the bar's 60-inch TV so people indoors can watch. By day, it stands at attention near a pool table. Terrill says the Bum Bot is promoting public safety.
"There are children in our neighborhood that use that day care center," he says. "People are coming on private property, they're defecating, they're throwing crack needles, sometimes they're throwing crack rocks."
Some who live on the streets say Terrill's going too far.
"This is going to intimidate a lot of people," says Rosetta Watkins, who used to live in a nearby shelter and has worked for Terrill. "You're going to intimidate a lot more people than you're going to help."
Homeless advocates agree.
"It's a play for public attention for Rufus. He's certainly got a lot of attention - but not the kind we need for housing, living wages," says Anita Beaty, director of Metro Atlanta Task Force for the Homeless. "This robot isn't casting attention to the deep, deep poverty in Atlanta."
Bulky as it is, the Bum Bot is surprisingly quiet. Terrill always stands about 10 feet behind it and follows a routine. First he turns on the red eyes, then the spotlight. Then he has the Bum Bot warn that it's videotaping the scene. Sometimes he has to call the police; usually people move on.
"I cleared out when I saw it," says Matthew Williams, a 23-year-old who lives in a nearby shelter. "People are starting to get used to it. People leave right when they see it."
Atlanta police spokeswoman Lisa Keyes says the department has not received any complaints about the Bum Bot but Terrill will risk charges if he intentionally sprays water on someone with the cannon or damages someone's property.
Terrill says he monitors only private property and never has blasted the cannon toward anyone. He also says he's employed 70 people from the streets and shelters since he opened the bar and helped 18 move into other full-time gigs.
Beaty is unconvinced of his motives.
"I'm appalled by the whole idea. It's a sham and a shame," says Beaty. "Rufus is using this for his 15 minutes."
Christensen at Georgia Tech is particularly concerned that the robot is a temporary - if entertaining - solution when Terrill could be helping find a permanent answer to vagrancy.
"He's moving the problem elsewhere," Christensen says. "And that works for him, but it's really not solving anything."
Terrill, for his part, seems stubbornly dedicated to resurrecting his neighborhood - in his way. Each night, he and the Bum Bot chase the same vagrants from the child care center. And each night, the men typically return within a few hours.
"This isn't fun," he sighs as he packs up around midnight. "I don't like being here every night. I'd be able to better run my business. But I have to spend all my time being the sheriff."