A Serial Infrared Remote Control Receiver
Note - this is a scan of a hand-drawn circuit.
Sorry about the low quality; I haven't gotten around to redoing it with CAD
This circuit is used to receive and demodulate an IR remote control signal.
The resulting data stream is then transmitted via an RS-232 port to whatever
system is waiting for it.
Overall Circuit Operation
The IR carrier state is sampled at periodic intervals, with the sampling
frequency high enough that the structure and specific values of the IR codes
can be determined.
The samples are stored in a shift register.
After each 8 samples have been stored, data from that shift register is
transferred to another shift register to be retransmitted via RS-232 at twice
the rate at which the signal is sampled.
The purpose of the deserialization followed by serialization is to allow the
transmission to occur at twice the sampling frequency, which in turn provides
“time space” for the RS-232 start and stop bits to be added.
Main Data Path
A GP1U252(1X), GP1U5(21X), or equivalent IR receiver module (manufactured by
Sharp, sold by Radio Shack at last check) receives and demodulates the 40 kHz IR
This gives a signal that is high when there is no carrier present and low when
carrier is present.
This is inverted (to high when carrier is present) by U1A, ¼ of a CD4093B (quad
NAND Shmitt trigger).
This is fed to an 8-bit serial-in, parallel-out shift register (both halves of
a CD4015B dual 4-bit shift register connected together to form an 8-bit shift
The parallel output of this is fed to 8 bits of a 16-bit parallel-in
serial-out shift register formed from two CD4014B 8-bit shift registers.
An input bit at one end of the PISO shift register is connected to logic low and
7 input bits at the other end of the shift register are connected to logic high.
These form the start bit and 7 stop bits respectively (only one stop bit is
needed, but making the entire frame be 16 bits simplifies the circuit design).
When the data is clocked out of the PISO shift register, these two “end” bits
form the RS-232 start and stop bits.
The output of the PISO shift register is fed to a MAX232 for translation to
RS-232 voltage levels, impedance, and slew rates.
Its output is connected to the TX pin of a 9-pin (or 25-pin) DSUB connector.
Transmitter Enable / Keepalive
The output of U1A (the demodulated IR signal, inverted so that carrier present
results in a logic high) is also fed to a retriggerable one-shot formed from a
diode, resistor, capacitor, and another Schmitt trigger.
The output of this will be low whenever an IR signal is being detected, and for
some time afterwards.
This is used to enable the RS-232 transmitter circuit for long enough after the
last IR data was received for that data to be fully transmitted before the
transmitter is disabled.
While the one-shot output is low, the transmitter clock is enabled.
When it is high, the transmitter clock is disabled, and further all of the
registers are cleared, ensuring that no spurious data is transmitted the next
time carrier is detected.
Transmitter Clock / Framing
The transmitter clock is a classic Shmitt oscillator formed from U1C.
Since it uses a NAND gate, it is disabled by bringing the extra gate input low.
It drives the PISO register and so sets the Baud rate of the serial output.
The values given are intended to be used to set a 38400 Baud data rate, and
the potentiometer should be adjusted to give a frequency of 38400 Hz.
Since the last data bit ends 9 bit-times after the start of the start bit,
and the last bit will be sampled by the receiver near the center of its bit
time, and each data frame is followed by an extended off time, the frequency
need only be accurate to within a few percent, making an RC oscillator
The transmitter clock also drives a framing counter formed from ½ of a CD4520
dual 4-bit counter.
Note that the enable input of this counter is being used as a clock input.
The 1 output of the counter, which gives the input clock divided by 2, in turn
drives the clock input of the SIPO register, causing it to sample the IR data
stream at ½ the rate at which data will be clocked out.
The rate using the described components and adjustment will be 19200 samples
per second, which gives about one sample for each two cycles of 40 kHz IR
carrier, and has been found sufficient for all remote controls examined.
Transmitting data at twice the sample rate leaves time for the start and
stop bits to be added.
When the framing counter reaches 15, the SIPO register will have been clocked 8
times and so will have loaded 8 IR samples.
At this count, U2A causes the PISO register to load all 16 of its bits,
transferring the 8 samples along with start and stop bits to the PISO register
to be transmitted over the serial line.