"Build a transmitter/receiver to implement the IrDA Optoelectronic standard at 115 kb/s. This will involve building a transmitter and receiver circuit for 115kb/s. The goal should be to transfer a file across a couple of meters of free space. A very good demonstration would involve the transfer of a file to an IrDA equipped printer or laptop."

Neville Kadwa
Thomas Lett
Beth Gemmell
Michael Welter
Jason Whittle

Our product consists of a pair of transceiver devices, each being capable of sending and receiving an electrical signal over the infrared spectrum. The transceivers are designed for point-to-point communication but are also capable of point-to-multipoint communications if multiple devices are within range.

To accompany our hardware, we also developed a Windows program to test and send a file between our transceivers. Our device will interface through a standard RS-232 serial port.

The basic optoelectronic components required are the LED at the transmitter and the photodetector at the receiver. IrDA specifications require the optical channel to operate in the near-infrared spectrum from 850 nm to 900 nm. The sensitivity of the photodetector is a function of the wavelength of the incident light.

To maximize efficiency, the LED was chosen so that its peak emission wavelength lies near the peak sensitivity of the photodiode. Beyond spectral matching, it is important when choosing an LED to consider other properties such as its radiation angle, radiant intensity, and rise and fall times. The output intensity of the LED is directly related to the amount of current flowing through it. We choose the SFH-205 as our LED. It meets the IrDA requirements, and is cheap and easy to find.

The choice of the photodetector requires more effort, because the performance is significantly affected by the interfacing circuitry. For high-speed optical communications, photodiodes are preferred, given their superior frequency response. For our desing, we choose the BPV22NF because it is rated in the infrared spectrum and is sensitive enough for our signal levels.

We chose to implement a simple Windows ME/2000 software interface for testing and demonstration purposes.

Windows offers an implementation of the complete IrDA protocol that can be used regardless of the underlying hardware.  This allowed us to create a simple program to test files sent between our transmitter and receiver.  For our demonstration, our program sends a file between two laptops equipped with our IrDA transceiver.

We chose Microsoft Visual C++ (VC++) as our program language for two reasons.  The Windows IrDA protocol uses standard Winsock API calls that are easy to access in VC++ and VC++ offers access to standard Windows GUI components.

The software is based around a thin-hardware philosophy. As long as the hardware is capable of sending and receiving an electrical signal, the software implements the IrDA protocols. These protocols are designed around the premise of the signal being converted into infrared and then back to electrical, but all they require is a communication pipeline from the send of one side to the receive on the other side.

As shown above, our program is simultaneously sending to and receiving from another laptop computer.