High Level Design

The system can send arbitrary data at 27.7kbps baud. The system illustrated above is set up to send voice data. The microphone input is conditioned (amplified and biased) so that the full eight bit range of the analogue to digital converter is utilized. A fair amount of time was spent optimizing the amplifier for the mic input so as to minimize popping but maximize quality.

Once the digital signal is obtained by the ADC, the MCU passes the signal to the UART. The UART sets a transmit pin high or low according to the serial protocol. Some conditioning is applied to this signal as well in order to ensure constant current to the laser.

On the receiver side, the signal is read by a photo transistor. The signal coming off the transistor is put through a comparator to generate appropriate high and low signals. The UART reads these signals and generates a byte according to the serial protocol. This byte is applied to a port, sent through a digital to analogue converter, and applied to a speaker.
2003 Keith Carter, Michael Muccio