Remote control is one such facility which enables the television viewer to operate from a distance (without leaving his or her seat) most of the controls situated on the front panel of the receiver.
Basic remote control system:
Figure 1.1 Basic remote control system.
- The transmitting unit may be mechanically actuated or electronic in nature.
- It generates ultrasonic sound waves at frequencies around 40 KHz. In the air these waves are directed towards the receiver where they operate the pick-up transducer which is normally a piezoelectric microphone.
- The use of frequencies around 40 KHz is advantageous because these are too high for the human ear.
- There are no standard ,some typical values for the main functions are:
41.25 KHz–Channel selection
37.75 KHz–Volume up
43.25 KHz–Volume down
Generation of Frequencies:
- A mechanical source or an electronic oscillator may be used to generate ultrasonic sound waves.
- The most common method employed to generate different frequencies by mechanical means is the use of vibrating rods.
- As shown in Fig. 1.2 the transmitting box (Bonger) contains several cylindrical aluminium rods of slightly different lengths.
- For each rod a separate control button is provided on the front panel of the hand-held remote control box.
- When any control button is pushed a spring loaded hammer strikes the corresponding rod and sets it into longitudinal mode vibrations at a definite frequency.
The sound energy thus generated is radiated through an open grille provided at the top of the box.
- At the receiver the ultrasonic energy is picked up by a crystal type microphone, which converts sound energy into corresponding electrical signals.
- The microphone employs a barium-titanate crystal element that has piezoelectric properties.
- A small bar of this material is placed between two conducting electrodes.
- When it is mechanically strained by the striking sound waves along one axis a proportionate voltage is generated along the other axis at the frequency of transmitted sound waves.
- A small ‘U ’ shaped piece of aluminium is added to the assembly.
- Beyond this assembly and at a distance of approximately one-quarter wavelength corresponding to a frequency of about 40 KHz, a small rectangular horn is attached.
- Both the one-quarter wavelength space and the horn serve to match the impedance of the barium titanate assembly to the air.
- The combination of this microphone and the amplifiers in the system provide sufficient sensitivity for operating various controls from a distance of about 7 to 10 metres.