This infrared audio transmitter uses infrared impulses modulated in time (pulse width modulation). Modulated signal is produced by comparing the pure audio signal with a triangular high frequency signal, using comparator Cl.
For a large range, the current through the LED must be high. Because LEDs can not drive continuous high current, pulses should be short, and it must be the reason for using pulse width modulation (PWM). Pulses are generated by XOR gate IC2d, which compare the original PWM signal delayed by R5-C3-IC2c.
The signal from the output of IC2d produce switching of T1. In this mode, the LED input current (with a supply voltage of 10 V) is limited to a peak of 400 mA, with the help of R6. Average current drawn by the circuit will then be about 90 mA.
Potentiometers P2 and P3 are used to eliminate any differences between the pulses generated by the leading edge and the trailing edge generated. P3's adjustment is made without the input signal using an oscilloscope, so that all pulses of the output signal to derive the same width. Then adjust P2 so that the intervals between pulses become equal.
P1 is used to obtain minimum interference in signal reception.