Datasheet

Jon Waddington
47
3.9 Infra-red Remote Controller
Infra-red (IR) remote controls are commonly used to control home entertainment
systems as they are simple to implement, very cheap to manufacture and make it easier
for the consumer to use devices as they can be operated from a distance. The main
disadvantage of infra-red is that the signal cannot penetrate opaque objects.
Remote controls use an IR LED to emit pulses of IR light. This is then re ceived by an IR
receiver which decodes the signal and executes the required function. To prevent noise
from interfering with the IR receiver from the sun or other sources of IR noise, the signal
is modulated onto a carrier signal, typically with a frequency of 36kHz. This means that a
burst of IR signals at 36kHz represents a logic ‘1’ and no signal represents a logic 0.
The protocol used in this project is the RC5 protocol. An RC5 code consists of 14 bits,
including 2 start bits which are always logic ‘1’, a toggle bit which toggles every time the
same button is pressed, 5 address bits and 6 command bits [38].
An RC5 transmission encodes the data in Manchester code. This method of encoding
ensures that there is a clock transition in the middle of each bit period; A low to high
transition is a logic 1 and a high to low transition is a logic 0 [39]. The bit period is
1.778ms long with a transition in the middle (after 889µs). The transition on every clock
pulse ensures that the receiver can synchronize with the transmitter more easily [40]. A
standard RC5 code is shown in Figure 3.9. It is transmitting the command value of 17.
This is the ‘decrease volume’ button.
Figure 3.9. A Typical RC5 Transmission
1
0
1
1
1
1
1
1
1
0
0
0
0
0
Start
Toggle
Address
Command
889µs
1.778ms