Operation Principle of the Transmitter Circuit: In this circuit a capacitive microphone is used to convert the audio signal into an electrical signal. The signal at the mV levels at the microphone output is applied to the op-amp after being amplified by the pre-boost circuit with bc 547. The signal at the op-amp output is modulated by laser beam applied to the base of the driver transistor connected to the darlington. 5V DC supply is used as the supply.
An important point here is that the laser wavelength we use as the light source is the same as the detection wavelength we use on the receiver side. The used Torx and Totx fiber optic transceiver sensors manufactured by Toshiba, which can provide both analog and digital communications with wave lengths of 650 nm. The sensors have a data transmission capacity of 15Mbit / s (this measurement is based on the NRZ code system, 7.5Mbit / s in the Bi-phase system).
Transmitter Circuit Diagram
Working Principle of the Receiver Circuit: The circuit uses the Torx147 receiver sensor to detect and convert the light signal from the fiber optic guide into an electrical signal. This laser receiver changes the amount of current passing through the incoming light intensity. The signal proportional to the amount of changing current is applied to the op-amp input via the c1 coupling capacitor. The amplified signal in the op-amp is applied to the output of the speaker or headphone to convert the electrical signal into an audio signal.
The most important elements in the receiver and transmitter are the receiver and transmitter sensors. It is not easy to find them on the market. We have bought the receiver and transmitter manufactured by Toshiban from abroad. I do not recommend the use of sensors without examining the necessary data, especially the receiver circuit can easily break down. It is much more advantageous to plug the sensors on the plaque than with the socket as we did.
Constant noise is present in the circuit. A transistor placed between the output of the receiver and the op amp will contribute to a stronger sound, but this will increase the noise ratio at the same time. The UA 741 OPAMP can be used to reduce noise, noise can be reduced by using other OPAMPs used for sound applications instead of sound. In order to reduce noises, a suitable filter for the output stage must be designed.
Receiver Circuit Diagram