Hardware

A typical system that SafetyNet would implement would consist of two types of device: a tracker and a reader.

Communication

RFID

RFID systems generally consist of a reader and a tag. A reader consists of an antenna, controlled transceiver and a logic interface with which it would communicate with other devices or networks. Tags tend to simply consist of an antenna and a chip, which stores particular information about the tag, including its ID. Antennae for tags can be as simple as a coil of wire, and are widely available at a low cost. The transceiver is able to transform the received signal into a recognisable code. As with the antennae, a wide variety is available at a low cost.

Short-Range Devices

An alternative to using RFID, which is a transponder based communication method, would be to use technology from Short-Range Devices (SRD). These are radio frequency devices used for various wireless applications ranging from car keys to hand held voice communication. Typically these have a much longer range than RFID but each device needs to be able to generate its own high frequency carrier signal. There are restrictions on the conditions under which these devices are allowed to operate, particularly the frequency and transmission power.

A typical long range RFID reader uses a mains power input, which restricts the possibility of a portable/mobile system. Use of a radio transceiver reduces the required power of the reader that would be used in RFID at the cost of increasing the power at the tag's end. However, the distance that a signal needs to travel is halved, which results in a better link budget at the same power. This suits our network much better, as the available power of each device is limited by a battery. The components required for this are a transceiver, antenna and microcontroller.

Processing

Microprocessor

In order to perform the distance calculations and signal processing required by our system, we need a reasonably powerful microcontroller that is easy to program. We also need to take power consumption into consideration, as well as price. We have therefore chosen the ATmega88PA-PU from ATMEL, which will be suitable for both the reader and the tags.

Power

Batteries

Since we wish to be able to transmit location data over long distances, we need to consider a small, long lasting power supply. While there are many batteries available which are cheap and long lasting, we came across an alternative solution known as a supercapacitor. These differ from normal capacitors in that they tend to have capacitance in the order of Farads, thousands of times greater than electrolytic capacitors. Unfortunately, they are not without disadvantages, and we believe that supercapacitors are not yet developed enough for us to use in our design. Instead, we have opted to use a coin cell battery, due to its long-lasting nature and voltage capability.