Operation Principle of the Transmitter Circuit:
In this circuit, an audio signal is converted into an electrical signal using a capacitive microphone. The microphone output, at millivolt levels, is amplified by the pre-boost circuit with a BC547 transistor and then fed into the op-amp. The output from the op-amp is modulated by a laser beam directed at the base of the driver transistor connected to the darlington configuration. The circuit is powered by a 5V DC supply.
Crucially, the laser wavelength used as the light source matches the detection wavelength employed on the receiver end. Toshiba’s Torx and Totx fiber optic transceiver sensors are utilized, capable of providing both analog and digital communications with a wavelength of 650 nm. These sensors offer a data transmission capacity of 15 Mbit/s (measured using the NRZ code system, or 7.5 Mbit/s in the Bi-phase system).
Transmitter Circuit Diagram
Principle of Operation for the Receiver Circuit: The circuit utilizes the Torx147 receiver sensor to detect and transform light signals from the fiber optic guide into electrical signals. This laser receiver adjusts the current flow based on incoming light intensity. The signal, proportional to the changing current, is passed to the op-amp input via the coupling capacitor C1. The amplified signal from the op-amp is then sent to the speaker or headphone output, converting the electrical signal back into an audio signal.
Critical components in both the receiver and transmitter are these sensors, which can be challenging to find in the market. We sourced our sensors internationally, opting for Toshiban-manufactured ones. I strongly advise against using sensors without thoroughly examining the necessary data, especially since the receiver circuit can be quite delicate. Attaching the sensors directly to the board, as we did, proves more advantageous than using sockets.
A constant noise is present within the circuit. Introducing a transistor between the receiver output and the op-amp enhances the sound, but this also escalates the noise ratio. To mitigate this, the UA 741 OPAMP can be employed, or other OPAMPs designed for sound applications can be utilized to minimize noise. Designing an appropriate filter for the output stage is crucial to reducing noise effectively.