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Remote control
extender for VCRs
This simple device will allow you to operate
your VCR via its IR remote control from
another room in the house. It works by
receiving the IR signal from the handpiece
& then retransmitting it to an IR LED near the
VCR via a 2-wire cable.
By JOHN CLARKE
Many families now have two colour
TV sets, one usually located in the
living room with a VCR and a second
set in the kitchen, rumpus room or
one of the bedrooms. But although it’s
quite easy to link both TV sets to the
VCR (via a 2-way splitter), operating
the VCR from the same room as the
second set is usually impossible.
This Infrared Remote Extender
solves that problem. It sits in the same
room as the second set and picks up
infrared signals from the VCR’s remote
16 Silicon Chip
control. This signal is then converted
to an electrical signal and sent down a
2-wire cable to an infrared LED located
near the VCR in the living room – see
Fig.1.
Because the signal from the infrared
LED mimics the signal picked up by
the receiver, the VCR will now respond
to any com
mands from the remote
control in the other room. Of course,
the extender is not only limited to
VCRs – it can be used to re-transmit
virtually any IR signal (eg, for CD
players or burglar alarms).
As shown in the photos, the circuit
for the Infrared Remote Extender is
housed in a small metal case. An
ACKnowledge LED on the front panel
lights whenever a signal is received
from the remote control, to let you
know that the unit is working correct
ly. There is just one control – an on/
off switch.
The rear panel of the device carries
two sockets, one for power (12V DC)
and the other to allow the cable for the
remote infrared LED to be plugged in.
Before moving on to the circuit description, we should briefly mention
the Infrared Remote Control Extender
published in the September 1990 issue. This proved to be an extremely
popular project but was not without
problems. Based on numerous enquiries from people who had constructed
the project, it was clear that the circuit
required some component adjustments (mainly around the AGC section) so that it would operate reliably
with a variety of infrared controllers.
This completely new circuit solves
the problems associated with the previous design.
How it works
Fig.2 shows the circuit schematic.
It uses an infrared photodiode (IRD1)
to receive the signals and a Plessey
SL486 infrared remote control preamplifier (IC1) to amplify these signals.
An elaborate AGC circuit based on
IC2a provides gain control for the am
plifier stage inside IC1, while inverter
stages IC3a-IC3e drive the infrared
and ACKnowledge LEDs (IRLED1
and ACK).
In greater detail, signals from
the remote control trans
mitter are
picked up by IR photodiode IRD1
and converted to electrical pulses.
These pulses are then filtered by a
twin-T filter with a notch frequency
of 100Hz to eliminate interference
from mains-powered lights and then
applied to the differential inputs of
IC1 at pins 1 and 16.
Normally, the twin-T filter is not
required since IC1 provides sufficient
attenuation at 100Hz when using its
recommended capacitor values to produce a roll-off below 2kHz. However,
we have altered the gain of IC1 at low
frequencies so that the roll-off begins
INFRARED
EXTENDER
INFRARED
LED
VCR
SECOND RECEIVER
MAIN RECEIVER
VCR
REMOTE
CONTROL
ROOM 1
ROOM 2
Fig.1: the basic concept. The IR extender picks up infrared light from the VCR’s
remote control & converts it to an electrical signal. This signal is then sent down
a 2-wire cable & drives an IR LED located in the same room as the VCR.
at 666Hz. This is to allow the circuit
to amplify signals from those transmitters with outputs centred on 1kHz.
The 22µF and 220µF capacitors
at pins 2 and 3 respectively of IC1
set the pi functions of two internal
gyrator circuits. In low ambient light
conditions, the gyrator circuit using
the 22µF capacitor is switched into
circuit, while in high light conditions,
the gyrator using the 220µF capacitor
takes effect.
The remaining capacitors at pins 5,
6 and 15 provide roll-off at frequencies
below 666Hz. This low frequency
roll-off works in conjunction with
Most of the parts are mounted on a small PC board & this must be fitted inside a metal case. Power
comes from a 12V DC plugpack supply.
April 1994 17
47
10
22
220
2
0.1
3
5
K
6
IC1
SL486
A
6.8k
6.8k
OUTPUT
16
REG IN
0.47
15
0.22
TP2
+6V
7
1
IRD1
BPW50
22
10k
4
14
12
AGC
13
680
.015
IC3a
74C14
IC3b
12
13
9
8
IRLED1
CQY89A
D2
1N4148
IC3d
100Hz NOTCH
3
100k
13
5
Q2
BC328
B
47
+6V
AGC
ADJUST
VR1
10k
FILTER
BUFFER
3
5
10
0.1
9
IC2d
8
100k
6
4
IC2b
7
2
2.7k
1
IC2c
INFRARED REMOTE EXTENDER
Automatic gain control
The automatic gain control (AGC)
output at pin 8 is normally connected
to a 0.15µF capacitor. This filters the
amplified signal and controls the gain
of IC1 to prevent signal overload.
Unfortunately, this AGC system
is only suitable for remote controls
which produce very narrow pulses of
infrared light. In most cases, however, the transmission code consists of
bursts of signal which can be anywhere
between 1kHz and 100kHz in frequen
cy. This type of coding produces too
much AGC for IC1, thereby rendering
the amplifier ineffective.
For this reason, we have completely
revamped the AGC circuit so that the
18 Silicon Chip
C
12VDC
INPUT
BUFFER
S1
1M
1000
16VW
D3
B
A
K
A
K
AMPLIFIER
Fig.2: each time an IR light pulse is received, pin 9 of IC1 switches high & drives
IRLED1 via IC3a & IC3b. A sample of the output pulse from pin 9 is also fed to
IC2a which works with IC2b, IC2c & IC2d to provide automatic gain control.
the 100Hz twin-T filter to provide a
high degree of attenuation for 100Hz
signals. If this were not done, noise
signals from mains-powered lighting
could degrade the receiver’s sensitivity
and reduce its effective range.
+6V
Q1
BC338
B
E
22
BP
DC OFFSET
-6V
TP1
+6V
E
C
3.3k
100k
11
0.1
14
IC2a
LM324
K
ACK
LED2
6 680 A
D1
1N4148
12
220k
IC3e
4
A
K
A
8
3.6k
POWER
LED3
680
IC3c
22
-6V
47k
10
7
9
.047
0.22
14
11
receiver will work with a wide range
of remote control transmitters without
the hassle of fiddly adjustments.
The modified AGC circuit works
as follows. First, the amplified output at pin 9 of IC1 is attenuated by
about 20% using a voltage divider
(47kΩ and 220kΩ) and applied to
the non-inverting input of op amp
IC2a. This op amp is connected as a
unity gain buffer and simply provides
current drive for an AGC filter consisting of D1 a 100kΩ resistor and a
47µF capacitor.
In operation, IC2a and the AGC filter
act as a peak detector for the output
signal that appears at pin 9 of IC1.
Each time a signal is received, the
47µF capacitor charges via D1 and is
then discharged by the 100kΩ resistor
so that the filter output decays after a
few seconds.
This filtered signal is applied to op
amp IC2b which operates with a gain
of 11, as set by the 1MΩ and 100kΩ
E
C
VIEWED FROM
BELOW
1N4004
ALL VOLTAGES MEASURED
WITH RESPECT TO GROUND
feedback resistors. The 22µF capacitor
across the feedback path filters the
output to provide the required AGC
response time.
Bias for the inverting input of IC2b
comes from the AGC adjust pot (VR1)
and is applied via unity gain buffer
stage IC2d.
IC2c and transistor Q1 together form
a high-current buffer stage for the output of IC2b. A 10kΩ pullup resistor
provides the collector load for Q1,
while feedback is provided from Q1’s
collector to the non-inverting input of
IC2c at pin 3. The buffer is made stable
by the 22µF capacitor at pin 8 of IC1,
the capacitor effectively slowing down
the open loop gain of the stage.
Because IC2c and Q1 operate with
unity gain, Q1’s collector voltage
follows the voltage fed to IC2c. Thus,
under no-signal conditions, pin 2 of
IC2c is at ground and so pin 1 goes
high and turns on Q1 (ie, Q1’s collector goes low). Conversely, when a
signal is received, the voltage on pin
2 rises and Q1 progressively turns
off. As a result, Q1’s collector voltage
12VDC
A
47uF
IC1
SL486
IC2
LM324
680
1
220uF
1
680
3.3k
22uF BP
680
Q1
LED2
K
1000uF
0.1
22uF
0.1
22uF
D3
D2
IC3
74C14
.015
3.6k
0.22
0.22
1
K
A
Q2
TP2
100k
A
K
IRD1
1M
LED3
2.7k
K
TP
GND
K
22uF
100k
10uF
VR1
0.1
47k
.047
6.8k
0.47
6.8k
220k
S1
100k
D1
K
K
LED3 LED2
A
A
10k
47
TP1
K
A
IRLED1
Fig.3: here’s how to wire up the IR Remote Extender. Take care with component
orientation & note that IRD1 is mounted with its leads untrimmed so that it can
be adjusted to line up with its viewing hole in the front panel. The board must
be fitted inside a metal case which is connected to the circuit via the solder lug
(at the top of the diagram).
rises so that it remains equal to the
voltage on pin 2.
The output from this buffer stage is
fed to the AGC pin (pin 8) of IC1. This
pin has a low input impedance but Q1
provides sufficient drive to overcome
the internal AGC level.
The AGC action works like this:
when the output signal from IC1 at
pin 9 exceeds the voltage preset by
VR1, the AGC voltage increases on
pin 8. This reduces the gain of IC1
and so the signal level is reduced.
Conversely, when the output from IC1
falls below the preset AGC voltage,
the AGC voltage at pin 8 falls and the
gain increases.
Signal drive
The resulting signal from pin 9 of
IC1 is squared up by Schmitt trigger
IC3a and inverted by IC3b. This then
drives the infrared LED (IRLED1)
via a 680Ω resistor. Thus, each time
a pulse of infrared light is received,
IC3b’s output switches high and
pulses IRLED1.
LED 2 (ACKnowledge) and its associated circuit provide visible indication that a signal has been received.
However, LED 2 cannot be driven by
IC3b because the pulses from this stage
are so short.
To overcome this problem, IC3a’s
output is inverted by IC3c and this
drives a pulse extender circuit consisting of diode D2, a 0.1µF capacitor
and 100kΩ resistor. Each time IC3c’s
output goes high, the 0.1µF capacitor
charges via D2 and buffer stages IC3d
and IC3e drive the ACKnowledge LED
via a 680Ω resistor.
Conversely, when IC3c’s output goes
low (ie, when no signal is being received), the 0.1µF capacitor discharges
via the 100kΩ resistor and the ACK
nowledge LED goes out. Thus, depending on the code from the transmitter,
LED 2 will flicker on and off but at a
much slower rate than IRLED1 due to
the time constant formed by the 100kΩ
resistor and the 0.1µF capacitor in the
pulse extender network.
Power supply
Power for the circuit is derived
from a 12V DC plugpack supply.
This is applied via reverse polarity
protection diode D3 and decoupled
by a 1000µF capacitor. Note that the
resulting supply lines have been
labelled +6V and -6V, rather than
+12V and 0V. This has been done to
simplify the supply labelling for the
rest of the circuit, particularly around
the op amps.
IC1 has an internal regulator which
RESISTOR COLOUR CODES
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
❏
No.
1
1
3
1
1
2
1
1
1
3
1
Value
1MΩ
220kΩ
100kΩ
47kΩ
10kΩ
6.8kΩ
3.6kΩ
3.3kΩ
2.7kΩ
680Ω
47Ω
4-Band Code (1%)
brown black green brown
red red yellow brown
brown black yellow brown
yellow violet orange brown
brown black orange brown
blue grey red brown
orange blue red brown
orange orange red brown
red violet red brown
blue grey brown brown
yellow violet black brown
5-Band Code (1%)
brown black black yellow brown
red red black orange brown
brown black black orange brown
yellow violet black red brown
brown black black red brown
blue grey black brown brown
orange blue black brown brown
orange orange black brown brown
red violet black brown brown
blue grey black black brown
yellow violet black gold brown
April 1994 19
gives about 6V between the positive
rail and ground. This 6V supply provides power for the rest of the circuit,
with the exception of the LEDs.
Transistor Q2 acts as a buffer stage
for the regulator ground supply. Its
emitter is effectively at ground (actually 0.7V), which means that the LED
currents flow through Q2 to the -6V
rail. This prevents the regulator inside
IC1 from being overloaded by the LED
currents (since these currents do not
flow to ground).
Finally, a 22µF capacitor is used to
decouple the 6V supply, while a 47Ω
resistor and a 10µF capacitor provide
additional supply line decoupling for
IC1 to prevent noise from affecting the
sensitive amplifier stages.
PARTS LIST
1 K&W metal case, 127 x 68 x
39mm
1 PC board, code 15303941, 59 x
115mm
1 self-adhesive label, 63 x 33mm
1 self-adhesive label, 63 x 11mm
1 12VDC 300mA plugpack
1 2.5mm panel mount DC socket
1 2-pin panel mount DIN socket
1 2-pin DIN line plug
1 SPDT toggle switch (S1)
2 5mm LED bezels
1 10kΩ horizontal trimpot (VR1)
4 9mm tapped standoffs
1 solder lug
4 3mm dia. x 15mm long screws
5 3mm dia. x 9mm long screws
9 3mm nuts
1 10-metre length 2 x 14/0.19 twin
cable
1 350mm-length twin rainbow
cable
1 120mm-length twin hookup wire
1 100mm green hookup wire (for
earth lead)
1 50mm-length 0.8mm tinned
copper wire
12 PC stakes
4 small rubber feet
Semiconductors
1 SL486 infrared preamplifier
(IC1)
1 LM324 quad op amp (IC2)
1 74C14, 40106 hex Schmitt
trigger (IC3)
1 BC338 NPN transistor (Q1)
1 BC328 PNP transistor (Q2)
2 1N4148, 1N914 signal diodes
(D1,D2)
1 1N4004 1A diode (D3)
1 BPW50 infrared photodiode
(IRD1)
1 CQY89A, LD271 infrared LED
(IRLED 1)
1 5mm green LED (LED 2)
1 5mm red LED (LED 3)
Capacitors
1 1000µF 16VW PC electrolytic
1 220µF 16VW PC electrolytic
1 47µF 16VW PC electrolytic
3 22µF 16VW PC electrolytic
1 22µF 50VW bipolar
1 10µF 16VW PC electrolytic
1 0.47µF MKT polyester
2 0.22µF MKT polyester
3 0.1µF MKT polyester
1 .047µF MKT polyester
1 .015µF MKT polyester
Construction
Most of the parts are mounted on a
PC board coded 15303941 and measuring 59 x 115mm. Fig.3 shows the
assembly details.
Begin the assembly by fitting PC
stakes to all the external wiring points
and to the three test points (TP1, TP2
& TP-GND). This done, install the wire
links, resistors and diodes. Be sure to
use the correct diode at each location
and make sure that it is correctly
oriented.
Now install the ICs, transistors and
capacitors. Note that the ICs are all
oriented in the same direction. The
22µF bipolar capacitor can be installed
either way around but take care with
the orientation of the remaining electrolytic capacitors.
Check the transistor type numbers
carefully when installing these parts
Resistors (1%, 0.25W)
1 1MΩ
1 3.6kΩ
1 220kΩ
1 3.3kΩ
3 100kΩ
1 2.7kΩ
1 47kΩ
3 680Ω
1 10kΩ
1 47Ω
2 6.8kΩ
Miscellaneous
Heatshrink tubing, solder,
insulation tape, etc.
+
+
+
ON
ACK
POWER
+
INFRARED LED SOCKET
(CENTRE ANODE)
INFRARED
REMOTE
EXTENDER
12VDC POWER INPUT (CENTRE +)
▲
Fig.4: here are the full-size artworks for
the front & rear panels.
20 Silicon Chip
Left: bend the leads of the infrared
photodiode (IRD1) so that its face lines
up with the matching front-panel cutout
but make sure that its leads don’t short
against the metalwork.
The infrared LED (IRLED1) is mounted at the end of the 2-wire cable. It can be
installed in a small case or taped in some inconspicuous location near the VCR.
Note that the anode lead of the LED goes to the centre pin of the DIN plug.
on the PC board. Q1 is an NPN type
while Q2 is a PNP type, so don’t get
them mixed up. Push the transistors
down as far as they will comfortably
go before soldering their leads.
The board assembly can now be
completed by installing the infrared
photodiode (IRD1). This device should
be mounted with its leads untrimmed
so that it can later be bent into position
to align with the hole in the front of the
case. Fig.1 shows the pin connection
details for photodiode.
Final assembly
A standard K&W metal case measuring 127 x 68 x 39mm is used to
house the PC board. Attach the front
and rear panel labels to the case (see
photos), then drill out the mounting
holes for the power switch (S1) and
for the Power and ACK LEDs.
The square cutout for IRD1 is made
by first drilling a small pilot hole and
then filing this to shape with a small
three-cornered file. This done, attach
a short piece of insulating tape to the
inside of the case beneath the hole to
prevent IRD1’s leads from shorting to
the metalwork – see photo.
Moving now to the rear panel, the
two sockets must be mounted high up
to provide sufficient clearance to the
PC board. Again, use small pilot holes
to begin with, then enlarge these to size
using a tapered reamer. A three-cornered file will be required to provide
the final shape for the DC socket. Once
the sockets fit their respective holes,
mark and drill the four holes for the
mounting screws.
The PC board is mounted in the case
on four 9mm-long standoffs. Use the
board as a template for marking out its
mounting holes, then drill these holes
to 3mm. You will also have to drill a
mounting hole for the earth solder
lug – see Fig.3.
Fig.5: check your etched PC board against this full-size artwork before installing
any of the parts.
Before installing the PC board in
the case, you will need to wire up
and install the power LED (LED 3).
Use twin rainbow cable for the LED
wiring and insulate the leads with
heatshrink tubing to prevent shorts to
the underside of the PC board. This
done, secure the earth solder lug to
the case and solder a short length of
hookup wire to it.
The PC board can now be installed
in the case and the wiring completed
using light-duty hookup wire. Check
your work carefully against Fig.3 to
prevent any mistakes.
The remote IR LED (IRLED1) is connected to the receiver via a long length
of light-duty speaker cable. This LED
can be either mounted in a separate
small case or taped to an inconspicuous location near the VCR. Be sure to
connect the anode lead of the IR LED
to the centre pin of the DIN plug.
Testing
To test the circuit, apply power from
a plugpack and check that the power
LED lights. Assuming all is well, check
the voltage between TP2 and the GND
terminal – the meter should read between 5.9V and 6.5V DC.
Next, activate the remote control
transmitter and check that the ACK
nowledge LED flickers when a button
is pressed. If it does, connect your
multimeter between TP1 and GND,
activate the remote control, and adjust
VR1 for a reading of 2V. This adjustment sets the AGC level.
The maximum range for the receiver can now be checked. This will
vary according to the remote control
transmitter but you should be able to
achieve at least five metres.
Finally, plug in the lead to the infrared LED and check that it correctly
activates your VCR in the other room
each time a transmitter button is
pressed. Note that the infrared LED
should be placed within one metre of
the VCR’s sensor for best results.
For some remote controls, you may
need to tweak the AGC level (using
VR1) to obtain the maximum range.
This should be done on a trial and
error basis, although the final setting
should not be too far from the setting
arrived at earlier.
In some cases, it may also be necessary to move the receiver away from the
TV set to prevent interference from the
line flyback pulses which can desensiSC
tise the front-end circuitry.
April 1994 21
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