The EM-18 is a low-frequency (125 kHz) RFID reader module, commonly used in applications like access control systems and inventory management. It reads data from RFID tags, transmitting the unique ID of the tag via UART or Wiegand interface. It's a compact module with a built-in antenna, designed for easy integration with microcontrollers and other systems. 

Key Features and Specifications:

·       Operating Frequency: 125 kHz.

·       Tag Compatibility: Commonly used with EM4100 or compatible 64-bit RFID tags.

·       Reading Distance: Typically 5-10 cm.

·       Interface: UART (serial communication) and Wiegand output.

·       Power Supply: 5V DC.

·       Dimensions: Small and compact, often around 32 x 32 x 8 mm.

·       Applications: Access control, inventory management, attendance tracking, library automation, etc. 

How it works:

1. Tag Proximity:

When an RFID tag (like an EM4100 tag) comes within the reader's read range (typically a few centimeters), the reader's antenna emits a radio frequency field. 

2. Tag Activation:

The tag, powered by the reader's signal, activates and transmits its stored unique ID to the reader. 

3. Data Transmission:

The EM-18 reader then sends this ID to a connected microcontroller or other system using either a UART serial interface or a Wiegand protocol. 

Advantages:

·       Easy Integration: The EM-18's compact size and simple interface make it easy to integrate into various projects. 

·       Cost-Effective: It's a relatively inexpensive RFID solution for many applications. 

·       Wide Compatibility: Works with common RFID tag types. 

·       No Line of Sight Needed: Unlike barcode readers, RFID doesn't require a direct line of sight between the tag and the reader.