Abstract

Given the freedom to pursue any particular electrical engineering design problem, we decided to implement an open-air, audio-optical, transmission-receiver system. With an audio input (a .WAV file containing digital audio information) we designed some signal processing software that would encode our data through a Manchester encoding scheme. The encoded data is then transformed into a voltage signal that modulates the intensity of the light coming from our transmitter (LED). The receiver (photodiode) measures the light intensity and reproduces the encoded signal, which we sample, then decode back into the original waveform. In theory, this waveform can be sent into a speaker to play the audio file we sent originally. We were successfully able to reconstruct a waveform similar to the one that was transmitted; however, due to lack of time, we were never able to fully implement playing that waveform through some speakers.

Problem Formulation

In a wireless market dominated by radio waves (cell phones, wifi, GPS, etc.), our device is not as widely applicable as other products since our receiver has such a small detection area. However, with our design, it is possible to transmit light signals at any arbitrary wavelength providing we utilize the proper receiver. This means that one can visually observe the datastream being sent in the form of a beam of light. Imagine a laser light show that is not only visually stunning, but simultaneously sending the information of the music you hear straight to the speakers that are playing it!

Given a laser transmitter, this device can also be used to demonstrate properties of lasers as well as measure the speed of light.

And as with all new technology, a problem could arise in the future such that our device might be utilized for more than just entertainment purposes.

Project Specifications

Modified Specifications:

Original Specifications (Now Future Implementations):

Concept Synthesis

Our original concept needed to transmit an audio signal from one point to another with minimal interference. A laser pointer was a logical choice, and Manchester encoding came recommended (above FM or other alternatives) from our advisor for its self-clocking properties.

Due to the complexity of a laser pointer’s circuitry, we elected to prototype our system using a white-light LED and a solar panel first. While this setup was capable of transmitting some signal, we found that it couldn’t respond at a high enough frequency to effectively broadcast a WAV file. Accordingly, we switched to an infrared LED with a photodiode (due to the relatively high availability of infrared photodiodes compared to others).

The intent was that we could extend our circuitry used in       this prototype to a system with greater range, but this is as far as we made it in our development.

      Figure: Decision tree,
      corresponding to our
      hardware and encoding
      scheme design process.

Design


Materials and Cost:

Figure Top: Block diagram of our prototype circuit. The Encoder handles the first two stages of our encoding scheme. The Decoder introduces the Delta Sampling encoding as a means of decoding the Manchester signal.
Figure Left: Example sequences from the original audio waveform and each of the stages of the encoding scheme.
Figure Below: The waveform before and after transmission.