Mechanism of Remote control in TV

In earlier days, remote controls were based on Ultrasonics. The controlling circuitry included a hand held transmitter and a TV-based receiver circuit. Electronic filters 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 rays and a special binary 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 injection laser diode (ILD). IT generates a code in a parallel format. This is converted to a series format by a 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.