An RFID (Radio Frequency Identification) card is a small, portable device embedded with an RFID chip that enables wireless communication for identification and access control purposes. These cards use radio waves to transmit data between the card and an RFID reader, making them ideal for secure access systems, inventory management, and time-tracking applications. Typically, RFID cards are used in various sectors, including security, transportation, and retail.

The RFID card only is designed to be lightweight and easy to carry. It can store unique identification data, which is accessed through a reader. There are two main types of RFID cards: Passive RFID Cards, which require no internal power source and are activated by the reader’s signal, and Active RFID Cards, which have a battery and can broadcast their signal autonomously.

How it Works:

1. RFID Communication: The card contains a microchip and an antenna that allows it to communicate wirelessly with an RFID reader. When the card is brought near the reader, it emits a signal with stored data, such as an ID number or other information.

2. Data Transfer: The reader captures the card’s signal, decodes the data, and sends it to a computer or access control system for authentication or processing.

3. Authentication and Access Control: The system compares the card's data against a pre-set database to grant or deny access. For example, an RFID card can be used for opening doors, making payments, or logging into a system.

4. Passive or Active: Passive cards do not require a battery and rely on the energy from the reader’s signal, while active cards have a battery to broadcast signals over a longer distance.

Limitations / Constraints:

1. Range: Passive RFID cards typically have a short reading range, usually up to a few centimeters or a meter, depending on the reader and environment. Active RFID cards have a longer range but may require battery replacement.

2. Security Risks: If the system is not well-encrypted, RFID cards can be susceptible to hacking or unauthorized duplication, posing a security threat.

3. Interference: RFID cards may experience interference from metal objects or other RF signals, which can affect communication reliability.

4. Privacy Concerns: As RFID cards can be read without direct contact, there are concerns regarding unauthorized tracking or monitoring of the cardholder.

5. Limited Data Storage: RFID cards typically store minimal data, making them suitable for simple identification tasks but unsuitable for storing large volumes of information.