In earlier days, remote controls wre based on ultrasonics (sound frequency above the audible range of frequencies). The controlling circuitry included a hand held transmitter 9that transmits ultrasound) and a TV-based receiver circuit. Electronic filter and stepper motors were used to allow/select certain frequencies and perform various functions depending on the key pressed.
But the recent remote controls use infra-red(IR) rays and a special binary (0 and 1) coding mechanism. The code, intensity and wavelength of the IR wave, help to select different functions. Depending on the key pressed, a signal is sent out by IR source say, an injetion laser diode(ILD). It generates a code in parallel format. This is converted to a series format by s shift register. This signal is received by photo-sensitive devices such as an
avalanche photo diode at the receiver.
Here another shift register is used to convert the code back to a parallel form. This operates a one-of-n decoder, which selects one function from a set of "n" predefined functions and executes it.
Courtesy : The Hindu
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