Application of Pulse Waveform

TV remote

The TV remote, or “clicker,” is so much a part of our modern-day living that we must all have wondered at one time or another how it looks inside or how it works. In many ways it is similar to the garage door opener or the car alarm transmitter in that there is no visible connection between the transmitter and the receiver, and each transmitter is linked to its receiver with a special code. The only major difference between the TV remote and the other controls is that the TV remote uses an infrared frequency while the other two use a much lower radio frequency.
The TV remote of Fig. 1(a) has been opened to reveal the internal construction of its key pad and face in Fig. 1(b). The three components of Fig. 1(b) were placed at a level that would permit matching the holes in the cover with the actual keys in the switch membrane and with the location that each button on the key pad would hit on face
Fig. 1: TV remote: (a) external appearance; (b) internal construction; (c) carbon key pads; (d) enlarged view of S31 key pad.
Note on the printed circuit board that there is a black pad to match each key on the membrane. The back side of the switch membrane is shown in Fig. 1(c) to show the soft carbon contacts that will make contact with the carbon contacts on the printed board when the buttons are depressed. An enlarged view of one of the contacts (S31) of Fig. 1(c) is shown in Fig. 1(d) to illustrate the separation between circuits and the pattern used to ensure continuity when the solid round carbon pad at the bottom of the key is put in place.
All the connections established when a key is pressed are passed on to a relatively large switch-matrix-encoder IC chip appearing on the back side of the printed circuit board as shown in Fig. 2. For the pad (S31) of Fig. 1(d), three wires of the matrix appearing in Fig. 1(b) will be connected when the corresponding key (number 5) is pressed. The encoder will then react to this combination and send out the appropriate signal as an infrared (IR) signal from the IR LED appearing at the end of the remote control, as shown in Fig. 1(b) and Fig. 2.
Fig. 2: Back side of TV remote of Fig. 1.
The second smaller LED (red on actual unit) appearing at the top of Fig. 1(b) blinks during transmission. Once the batteries are inserted, the CMOS electronic circuitry that controls the operation of the remote is always on. This is possible only because of the very low power drain of CMOS circuitry. The power (PWR) button is used only to turn the TV on and activate the receiver.
The signal sent out by the majority of remotes is one of the two types appearing in Fig. 3. In each case there is a key pulse to initiate the signal sequence and to inform the receiver that the coded signal is about to arrive. In Fig. 3(a), a 4-bit binary-coded signal is transmitted using pulses in specific locations to represent the “ones” and using the absence of a pulse to represent the “zeros.” That coded signal can then be interpreted by the receiver unit and the proper operation performed.
Fig. 3: Signal transmission: (a) pulse train; (b) variation.
In Fig. 3(b), the signal is frequency controlled. Each key will have a different frequency associated with it. The result is that each key will have a specific transmission frequency. Since each TV receiver will respond to a different pulse train, a remote must be coded for the TV under control.
There are fixed program remotes that can be used with only one TV. Then there are smart remotes that are preprogrammed internally with a number of remote control codes. Remotes of this type simply need to be told which TV is involved using a three-digit coding system, and they will adapt accordingly. Learning remotes are those that can use the old remote to learn the code and then store it for future use. In this case, one remote is set directly in front of the other, and the information is transferred from one to the other when both are energized. Remotes are also available that are a combination of the last two.
The remote of Fig. 1 uses four AAA batteries in series for a total of 6 V. It has its own local crystal oscillator separate from the IC as shown by the discrete elements to the top right and mid-left of the printed circuit board of Fig. 1(c). The crystal itself, which is relatively large compared to the other elements, appears on the other side of the board just above the electrolytic capacitor in Fig. 2. It is the responsibility of the oscillator to generate the pulse signal required for proper IC operation. Note how flush most of the discrete elements are in Fig. 1(b), and note the rather large electrolytic capacitor on the back of the printed circuit board in Fig. 2. The specifications on the unit give it a range control of 25 ft with a 30° coverage arc as shown in Fig. 4.
Fig. 4: Range and coverage arc for TV remote of Fig. 1.
The arc coverage of your unit can easily be tested by simply pointing it directly at the TV and then moving it in any direction until it no longer controls the TV.