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Introduction

This project is intended to investigate and construct a system capable of communicating with a smart card in a room, without the need for contact or holding the card still while communication occurs. A data transport layer has already been developed, so this project will focus on the optical system and filtering necessary to complete a physical communications layer.

The Original Plan

We planned to use an infrared emitting strip light used for security cameras as a light source. The smart cards would contain a retro-reflector (an optical device that reflects incoming light in the direction from whence it came) and a liquid crystal shutter to modulate the returning light beam.

This system has a number of challenges: it is a low-bandwidth system, with extremely low signal levels, and a high background noise level (typically from fluorescent lights).

The diagram below shows the system in use.

However, this process proved to have light levels that were simply too low to practically detect. While the link budget that I created implied that it would be possible, a number of additional factors that I did not consider may have made an impact on this:

Loss due to the LCD shutter and its polariser;

Imperfections in the retro-reflector;

Electrical background noise inherent in the photodetector.

Due to this, we have decided to take an alternative route.

Plan B

In place of the infrared lighting strip, we will use a tungsten spotlight. This restricts the acceptance angle of the receiver, but this should not be a significant problem if multiple receivers are used. This light source provides significantly more light power for the detector. We will also be using a more efficient LCD shutter.

Using this new system, we have successfully detected a 300Hz modulation at 1.5m. We hope to achieve a bit rate of about 1kbit/s, which is within the new LCD's bandwidth of 7kHz.

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Optical Smart Cards

Introduction

The Original Plan

Plan B

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