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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