Resumen
The letter is a means of communication to convey information in writing by one party to the other. Its functions include some things that as a means of notices, requests, thoughts, and ideas. In the modern era, as now the letter has been abandoned by many users because it is considered less effective. However, in some cases letters are still a very important role. That requires a tool that can support the function of the letter in order to be more effective. The design of this tool requires several components, among others: Photodioda Sensor, Microcontroller AT89S51, and Mobile. Photodioda sensor is used to detect incoming mail and then send the data and the sensor will be processed by a microcontroller AT89S51 Micro kemundian giving commands for sending messages to mobile phones. Letter Post Paraphernalia Information-Based SMS Gateway can be used to provide information to homeowners provide facilities to determine whether there is an existing letter in the mailbox.Keywords: Letters, Microcontroller AT89S51, photodioda sensor, mobile