IR Remote extender
This project describes how
to build an IR remote control extender / repeater to control your electronic
appliances from a remote location.
An IR detector module receives
IR signal from remote control and two IR leds are re-emitting the signal to the
appliance. You can place the IR emitting leds close to the device you would
like to control using some wire and keep main unit close to remote control
location. In the image at the left leds are soldered on the board. The circuit
consists of three main parts, the IR receiver module, a 555 timer configured as
an oscillator and the output / emitter stage. We will describe circuit
operation below.
IR Signal
The IR signal emitted from
a remote control caries the information needed to control the appliance. This signal
consists of pulses that code 0 and 1 bits, instructing the appliance to do a
certain operation. One of the most common proto
cols used to code the IR signal is
Philips – RC5 protocol.
The signal consists of two parts, the control pulses and the carrier wave as
seen in the image below.
A common frequency used
for the carrier is 38KHz and control pulses frequency is in the range of
1-3KHz. The carrier signal is modulated by the control pulses and the resulting
signal is emitted by remote in IR band of electromagnetic spectrum. IR band is
invisible to human eye. You can see if an IR led is emitting light or not using
a camera. Point the camera to the led and you will see that light comes off.
Circuit description
IR signal is received by
TSOP1738. TSOP1738 is an infrared receiver at 38KHz. At the output of infrared
receiver we get a demodulated signal that means we get the low frequency
control pulses. Infrared receiver is powered from C1, R1 and Z1 that forms a 5V
power supply. With no signal received, infrared detector output is high and Q1
is on, so pin 4 of IC is LOW and 555 timer is in reset state. Q1 also acts as a
level shifter that converts 5V signal of TSOP1738 to 9V signal for IC1.
When HIGH control pulses
are appearing on TSOP1738 output then timer 555 (which is configured as an
oscillator) starts to oscillate are a preset frequency, for the duration of each
data pulse. That means that at pin 3 we get a signal that is similar to modulated
source signal. It has a carrier component and a control pulses component.
Oscillating frequency of 555 timer is set by R4 and C2 and pulse period is
given by:
T = 1,4 R4 C2
Trimmer R5 is used to fine
tune oscillating frequency at 38KHz. That’s equal to carrier frequency.
The output stage is formed from R6, Q2, one red LED, two IR
LEDs and two current limiting resistors R7 and R8. Q2 is connected as voltage
follower, that means when base of Q2 is HIGH transistor is ON allowing current
to flow through LEDs. LED current is set by R7 and R8 according the following
formula:
So IR
LEDs are emitting a signal that is similar to the signal received by TSOP1738,
that means it repeats the signal received at higher infrared radiation intensity.
The red LED is used as an optical indicator of output signal. Circuit can be
powered from a 9V battery.
Parts List
R1 = 1k
R2 = 3k3
R3 = 10k
R4 = 15k
R5 = 4k7 trimmer
R6 = 2k2
R7 = 470R
R8 = 47R – 1/2W
C1 = 47uF – 16V
C2 = 1n - polyester
C3 = 100uF – 16V
C4 = 47uF – 16V
Z1 = 5V1 zener
Q1 = BC549C
Q2 = BC337
IC1 = NE555
LED1 = red LED
LED2,3 = IR LED
IR receiver = TSOP138 or IR38DM
PCB
PCB is designed using Cadence Eagle.
Testing
Before powering the
circuit, remove IR LEDs. With no input signal red LED should be off. Now press
a button on a remote control, red led should flicker. If that’s the case then
your circuit should be working ok. Install IR LEDs. We found during testing
that IR signal emitted from remote and IR signal emitted from circuit are
interfering each other and that’s make receiving device not to react on
receiving the signal, this happens when IR from remote and IR from circuit’s
LEDs are on the same room. To solve that we must isolate the IR beam of remote
control. To do that we used a thin pipe in front of infrared sensor as seen in
photo below, so that the beam emitted from remote hits the sensor directly.
Another solution to this would be to put the emitting LEDs on a different room.
Installation
We installed the circuit
on the wall the way you see on the photo below. You can see that remote control
led is optically isolated from surround. You can also notice that one LED is
remotely placed near the device we would like to control.
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