125kHz LF (Low Frequency) RFID cards are passive identification devices widely used in access control and security applications. These cards operate without batteries, instead drawing power through electromagnetic induction when brought within range of a compatible reader.

The card contains a simple microchip and an antenna coil, typically made of copper wire with multiple turns. When the card enters a reader's magnetic field, the induced current powers the chip, which then transmits its unique identification number back to the reader through load modulation.

Key characteristics of 125kHz RFID cards include:

• Read range: Usually 2-10 centimeters
• Data capacity: Generally limited to a fixed 32-128 bit ID number
• Data transfer rate: Relatively slow compared to higher frequencies
• Cannot be reprogrammed (read-only)
• Not encrypted, offering basic security only
• Highly reliable in challenging environments
• Can work near metal or liquid
• Low cost and durable construction

Common formats include:

• ISO card (credit card size)
• Key fob
• Adhesive disc tag

These cards are particularly popular in:

• Building access control
• Time and attendance systems
• Legacy security systems
• Parking facilities
• Hotel room keys
• Pet identification

While newer technologies like 13.56MHz HF RFID offer better security and functionality, 125kHz cards remain widespread due to their established infrastructure, reliability, and cost-effectiveness. Their simplicity and durability make them suitable for basic identification tasks, though they are gradually being replaced in applications requiring higher security or data storage capabilities.

The technology is considered mature and standardized, with various manufacturers producing compatible cards and readers, ensuring broad interoperability across different systems and installations.