Low frequency RFID work at 125 KHz frequency with radio waves. There is a coil inside the RFID tag and when it is influenced by a magnetic field, it sends a 12 byte identity code to RFID reader for further processing. (For more details, refer interfacing RFID with AT89C51).
The RFID tag is used as a unique identity for account of a particular user. When a vehicle drives through the toll plaza, its driver is prompted to scan his RFID tag. If the identity (serial number of the tag, i.e., 12 byte data) is matched with the one already stored in the system, the toll amount is deducted from his account. After this, the vehicle gets immediate access to drive through.
This RFID based toll system also has some additional features. A new user can register him with the system. Also an old user can recharge his account balance. The amount for recharge can be entered in the system through a numeric keypad interfaced with AT89C51.
In beginning, the user is prompted to scan his tag or ID. The serial code of the tag is identified by the reader module and is sent to AT89C51 for comparison with stored data. If the ID is matched by the microcontroller, the toll amount (in this case Rs.10) is deducted from user’s balance and user gets to drive through the plaza. On the contrary, if the tag is not identified, a message (‘Wrong ID’) is displayed on LCD screen.
A new user needs to press the ‘*’ switch to register himself after which his identity is verified with RFID tag. The new record is then stored by the microcontroller to grant future access. The system also shows ‘Error’ if the tags do not match during verification. If balance in user’s account becomes less than Rs.30, a warning message ‘Low balance’ is displayed on LCD. In this case the user still gets the access through the system. The system also prompts the driver to recharge the account. If balance reaches below Rs.10, ‘Insufficient balance’ is indicated on LCD screen. The user does not get access until he adds more balance to his account. The recharge amount is entered by using a keypad. Back space can be used while entering the amount by pressing ‘*’ button on keypad. It also has a safety feature that if different tags are scanned through the reader at the time of registration, LCD displays ‘you have shown different ID’.
When an RFID tag comes in this range, the reader detects it and sends a unique code of the tag serially. This serial code, consisting of 12 bytes, is received by the microcontroller. This code is the ID for the user and is stored as an array in the microcontroller. If the ID is matched with this code stored in array of microcontroller, the user is granted access though the toll plaza. For more details on working and connections of the RFID, refer RFID interfacing through serial interrupt.