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
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
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: 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.
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