Infra-red transmitter: It often happens that someone wants to watch television without the sound disturbing other people in the room. To avoid having a long cable trailing over the floor, the present transmitter (and accompanying receiver ) make it possible to listen to the TV sound wirelessly.
The transmitter uses pulse-duration (=pulse-width) modulation. The modulated signal is produced in the traditional manner of having the audio signal set against a pure high-frequency triangular signal in a comparator. A suitable triangular signal generator can be found in Chapter 6. If another generator is used, make sure that its offset is equal to half the supply voltage of 5 V and its peak value is 2.5 Vpp.
For a good range, the current through the LEDs must be large. Since, however, LEDs should not draw large continuous currents, the pulses must be short, and this is why PDM is used. (In this type of modulation, the time of occurrence of the ‘ first and last transition, that is, the leading and the trailing edge, is varied from its ! unmodulated position).
The pulses are generated by XOR gate IC1, which compares the original PDM signal with one delayed by R5-C3-IC2. This results, for every level change, that is a transition, into a short pulse whose width is equal to time constant R5-C3.
The output signal of ICid switches Tl. In this way, the current drawn by the LEDs 1 (with a supply voltage of 10 V) is restricted to a peak value of 400 mA with the aid of R6. The average current drawn by the circuit is then some 90 mA.
Presets P2 and P3 serve to remove any differences between the pulses caused by a leading edge and those resulting from a trailing edge. In the absence of an input signal. set P3, with the aid of an oscilloscope. so that all pulses in the output signal have the same width. Then set P2 so that the pulse spacings are equal.
The output of IC1 should then be a perfect square wave.
With the receiver used in combination with the transmitter (with maximum input), set P1 to obtain the lowest possible interference on the received signal.
Finally, note that IC2a and IC2b may be used for the triangular wave generator.