tailieunhanh - Practical Arduino Cool Projects for Open Source Hardware- P30

Practical Arduino Cool Projects for Open Source Hardware- P30: A schematic or circuit diagram is a diagram that describes the interconnections in an electrical or electronic device. In the projects presented in Practical Arduino, we’ve taken the approach of providing both a photograph and/or line drawing of the completed device along with a schematic. While learning to read schematics takes a modest investment of your time, it will prove useful time and time again as you develop your projects. With that in mind, we present a quick how-to in this section | C H A P T E R 14 RFID Access Control System RFID technology pronounced Arr-Eff-Eye-Dee or Arr-Fid is used for a wide variety of applications including access control package identification warehouse stock control point-of-sale scanning retail antitheft systems toll-road passes surgical instrument inventory and even for identifying individual sheets of paper placed on a desk. RFID tags are embedded in name badges shipping labels library books product tags and boxes installed in aircraft hidden inside car keys and implanted under the skin of animals or even people. RFID systems work on a wide range of frequencies have a variety of modulation and encoding schemes and vary from low-power passive devices with range of only a few millimeters to active systems that work for hundreds of kilometers. With such a vast range of applications and related technologies it s no wonder that most people are confused about what RFID actually is Part of the problem is that the term RFID is a generic label for any technology that uses radio communication to check the identity of an object. All sorts of radically different systems fall under the broad banner of RFID. However all RFID systems have the same basic two-part architecture a reader and a transponder. The reader is an active device that sends out a signal and listens for responses and the transponder the part generally called the tag detects the signal from a reader and automatically sends back a response containing its identity code see Figure 14-1 . Figure 14-1. Reader challenge and tag response One of the earliest RFID systems was developed in England in 1939 to solve the problem of Allied aircraft being targeted by friendly antiaircraft guns as they returned to base. Unfortunately friendly aircraft returning home look pretty much the same as enemy aircraft approaching to attack so radio 269 CHAPTER 14 RFID ACCESS CONTROL SYSTEM transponders called IFF systems Identification Friend or Foe were installed that would allow .