Purpose of Infra-red headphone transmitter:
The proposed transmitter provides an optical (infra-red) that is wireless, connection to a headphone.
Three infra-red (IR) LEDs are provided with a quiescent current by T1. The level of that current is set with P1. When an audio signal is applied to the gate of T1, the current through the LEDs is modulated. Consequently, the light emitted by the diodes is also (amplitude) modulated.
To prevent overdriving of the gate causing too high a current through the LEDs, a current limiter, consisting of T2 and R3, holds the current below 100 mA. The maximum dissipation of a BS170 is 830 mW at an ambient temperature of 25oC, while the maximum drain current is 500 mA. Therefore, even when the FET is overdriven, those limits are not exceeded.
The optimum quiescent current through the LEDs must be determined in conjunction with the receiver (which must be adjusted for minimum distortion). The prototype transmitter drew a current of about 60 mA at a supply voltage of 9 V. It is, therefore, advisable to use a mains adaptor, because that current is just a little too high for a PP3 battery. Keep the earths of the mains adaptor and the audio signal separated as shown in the diagram to prevent feedback of the LED current to the input.
The gate-source voltage of a BS 170 may be up to 15 V. If you use a signal source that delivers a higher level, it is advisable to incorporate a simple protection circuit (for instance, a 10-V zener diode parallel, and a resistor in series, with the input).
The optical connection is fairly directional, but this can be improved by placing the LEDs at varying angles. Also, the distance of operation can be extended appreciably by fitting reflectors behind the LEDs. The optimum input level for an operating distance of some meters (4-8 ft) is 100-200 mV.