The extra shifts that D.C. police Officer David J. Cephas volunteered for proved lucrative, authorities said, allowing him to rack up more than $17,000 in overtime in a single year by running mobile radar and writing speeding tickets.

But the 12-year veteran pocketed the extra money while neglecting a crucial part of his job, according to authorities: He failed to conduct hourly tests to calibrate the digital units, then falsified logbooks to cover his tracks, forcing D.C. officials to nullify 200 citations and refund $17,550 to motorists caught in the traps.

On Thursday, Cephas pleaded guilty in D.C. Superior Court to three misdemeanor counts of fraud. He is scheduled to be sentenced Aug. 30, when he will face up to 180 days in jail and a $1,000 fine on each count.

As part of his plea, he resigned from the department in June and agreed to pay back the money the District had to return to drivers ticketed with radar guns that police could not say were giving correct readings.

Cephas’s attorney, Rebecca S. LeGrand, declined to comment.

D.C. officials said Cephas’s admitted scheme — in which he “rolled back” time stamps on the digital readouts, much like rolling back mileage on an odometer — cost the city tens of thousands of dollars in wasted overtime, the cost of 40 hours of training and the forfeited fines.

In a plea agreement, prosecutors outlined a deliberate scheme. Aside from getting paid overtime while failing to complete his job, it does not appear from the court filing that Cephas had a further financial incentive.

“The whole thing is laziness,” D.C. Police Chief Cathy L. Lanier said in an interview. “For whatever reason, he forgot, wasn’t paying attention or fell asleep. And rather than be honest, he did this. It seems like a very costly decision.”

In court papers, authorities said Cephas’s scheme lasted from May 1, 2008, through May 30, 2009, when it was uncovered during an audit. They did not say whether the audit was routine or the result of a tip or complaint.

Tickets issued on radar equipment that is not properly calibrated can be thrown out at trial, and prosecutors said in court filings that the logbooks in which Cephas recorded test results were fabricated.

The court documents say that members of the police department’s Automated Traffic Enforcement Unit were at that time required to test the radar units at the beginning and end of their shifts and every hour in between. Officers who failed to do so faced a 90-day suspension from the overtime detail.

Prosecutors said that over 13 months, Cephas submitted 33 falsified deployment logs. For example, court papers say, Cephas failed to take test photos at 2 a.m. and 3 a.m. on Dec. 7, 2008. At 3:10 a.m., the plea agreement says, Cephas “took the mobile radar unit and physically rolled the clock back to 2:01 a.m. before firing a test shot. Within a minute of taking the test shot, the defendant then rolled the clock forward to 3:01 a.m. and took a second test shot.”

Lanier said “test shot” calibrations are now done automatically.

“After this came to light, we put controls in place so this would not happen again,” the chief said.