The ADT Corporation, which provides home and business security systems for 6.5 million customers, is being acquired by the private equity firm Apollo Global Management for $6.9 billion in the largest leveraged buyout so far this year.

Apollo intends to merge ADT with Protection 1 and ASG Security, two other security companies that the firm agreed to purchase last May, although they will all operate under the ADT name. The combined company would create pro forma annual revenue surpassing $4.2 billion.

ADT traces its roots to Edward A. Calahan, the man who invented the stock ticker in 1867. When Mr. Calahan, the president of Calahan’s Gold and Stock Telegraph Company, found a burglar in his home, he created a telegraph-based alert system. After more than a century of acquisitions and antitrust rulings, ADT was acquired by Tyco in 1997. Tyco decided to be broken up into three units in 2012, making ADT once again an independent company.

ADT was the last of Tyco’s largest former subsidiaries to be acquired. The company first merged its flow-control unit with Pentair in 2012. In January, the security and fire-safety systems company that maintained the Tyco name decided to combine with Johnson Controls.