Many IR receivers add more complex circuitry to unlike the dashes and dots of Morse code. In Sony’s
comb out spurious signals prior to bit detection. scheme, 1 is represented by a 1,100-microsecond
These include tuned filters that select (bandpass) (µs) burst of 40 kHz light, and 0 is represented by a
and reject (notch) particular frequencies, and 550µs burst.
synchronous detection circuits, which multiply the Data streams over the IR link in LSB-first order
signal by the carrier frequency you are looking for. (LSB = least significant bits), with an extended
Because sine waves at unrelated frequencies cancel start bit of 2.4-millisecond ( 2,400µs) duration to
each other out overall, this multiplication results in indicate the beginning of a signal transmission. The
only the carrier frequency and its harmonics being extended start bit allows IR receivers to save power
left with a nonzero result. by running idle during periods of no transmissions,
Detector modules built to support the 40kHz then have enough time to wake up and watch when
standard also make great sensors for object detec- there’s a start signal.
tion in robotics, because of their robust handling of Each key-press event on a remote sends an IR
spurious signals. data packet consisting of two parts: a 7-bit com-
mand followed by a 5-bit device address. The
IR Protocol command encodes the identity of the key that was
Using a carrier frequency will solve the general pressed, and the device address specifies what
problem of spurious signals, but you still need a type of appliance the key press was meant for. For
way to encode data for transmission. Sony remote example, the power button is key number 21, a TV
controls use what’s called pulse-duration modulation, is at device address 1, and a CD player is at device
where the value of a bit is communicated by the address 17.
duration of a pulse of the carrier frequency — not
False IR Signals
A. Bright, shiny can logo D. False binary signal
B. Path of reflected light E. Equivalent bar code
C. On/off points for
false binary, from
reflections off can
IR protocol is designed to prevent the possibility of
spurious signals, but in the noisy, light-filled world,
there are unlikely instances where an IR receiver
might detect something that looks like binary data.
Consider this pathological case (of soda): Someone
in the room moves a shiny, printed aluminum can
near a window in the sun. Sunlight reflects off the
can, and bounces directly into the IR receiver. The
path of incidence of the beam hitting the sensor
also happens to travel across a two-color logo. One
of the logo’s colors absorbs IR, while the other
color reflects it. As a result, the sensor gets a short
blast of fast-pulsing IR.
With a sensor module that combs everything
it sees for binary patterns, this random reflection might appear to be a signal from a remote.
Hopefully it doesn’t look like the dreaded “
It’s also possible to intentionally create reflective
patterns that you shine light across and read as
data. This is how bar codes work.