This infrared transmitter can be used with this infrared receiver. This transmitter uses. The modulated signal is produced by comparison of an audio signal with a triangle signal of high frequency using a comparator IC. An appropriate triangle signal generator must have the offset equal with half of the 5V power supply voltage and the triangular signal size is 2.5V. Check out the infrared alarm circuit too.
For a wider range of the infrared transmitter the current through LEDs must he high. Because the LED cannot stand high dc currents, the impulses must be short so we use PWM. The pulses are generated by XOR gate IC2d that compares the original PWM signal with the delayed by R5-C3-IC2c. The IC2d output signal produce T1 switching and in this way the LEDs absorbed current is limited at 400 mA peak with help from R6. The average current consumption of the infrared transmitter circuit will be around 90 mA.
P3 adjustment is made without input signal with help of an oscilloscope, so that all impulses from the output signal have the same width. Then adjust P2 so that the intervals between impulses come equal. IC1 output will then be a perfect rectangular wave.
Working with the receiver and the transmitter at the same time feed an input signal at maximum value then adjust P1 to obtain minimum interference in the received signal. Finally, you may use IC2a and IC2b for the triangle signal generator.