I, like many people,
have a second television set in the bedroom, which is connected to the video
and satellite equipment downstairs.However, the pleasure of watching TV while lying in bed is lost by having to go downstairs to stop the video or change channel on the satellite receiver.
This project allows you to take the video recorder and satellite receiver remote controls upstairs, and operate the equipment from there. There is no additional cabling to install, the signal being carried along the existing coaxial aerial cable linking the two rooms.
The unit is in two sections, the infrared receiver which lives upstairs by the TV, and the infrared transmitter and power supply which lives downstairs and points at the equipment to be controlled.
The prototype has been tested and found to work reliably with about 50 metres of cable. Most domestic cable runs are much shorter than this, typically 10 to 20 metres. Indeed with 50 metres of cable the picture quality at the far end would probably be fairly poor!
Circuit Description
For now, assume that the two sections of the circuit are connected directly (SK1 joined to SK3).
Infrared Receiver
The infrared receiver circuit is shown in figure 1A.
D1 is the IR photo-diode and IC1 (TBA2800) is the infra-red amplifier. This IC contains three stages of amplification, the first of which has an automatic gain adjustment system to cope with varying signal and ambient light levels. The second amplifying stage simply provides further amplification, and the third separates the wanted signal from the general background noise. An inverting stage is also provided to give both positive and negative outputs.
The overall gain of the IC is quoted as 70dB, and the typical current consumption is 1mA at 5V.
C3 and C4 are the coupling components between the amplifying stages. The values of these have been chosen to give good coupling at the IR transmission frequency, while rejecting lower frequency noise and interference. C2 is the filter component for the automatic gain control of the first amplifier in U101. The power supply to IC1 is decoupled by R1, C1 and C5.
The inverted output of IC1 is connected TR1 (BC558), which in turn drives TR2 (BC548).
TR2 connects the LED D3 across the power input to the circuit. The purpose of this is to cause pulses of increased current consumption in time with the received infrared, which are in detected by the other section of the circuit. The LED flashes in time with the received infrared.
The circuit is powered from the other section of the circuit via SK1. D2 and R15 provide a regulated 5V supply to IC1, while D4 and C1 ensure that this supply does not vary significantly when the LED is pulsing.
Infrared Transmitter and PSU
The infrared transmitter and power supply circuits are shown in figure 1B.
The variations in supply current to the receiver section cause a varying voltage drop across R14. This is converted to logic pulses by TR5 (BC558). C12, R10 and D9 cause short (40uS) pulses to be applied to the base of TR4. TR4 (BC548) and TR5 (ZTX650) are in a Darlington arrangement, and drive the infrared LED.
The infrared LED (D7) has a maximum continuous current rating of 100mA, which would give a range of only a few centimetres. However the device has a pulse rating of over 2A, providing the duty cycle is short and the mean current does not exceed 100mA.
This gives a much improved range and is the technique used in commercial remote controls, as well as this unit. C11 acts as a reservoir for the LED current, and is charged when the LED is not lit via R13. The current to D8 is limited to about 2.5A by R9; a red LED (D7) and series resistor (R8) are connected across R9 to give a visual indication that the unit is operating.
The circuit is powered by a small transformer, giving an unregulated supply of about 18V across C10. The supply reaching the infrared receiver section will be about 12V. A 100mA transformer is adequate since the current consumption is only a couple of milliamps when the unit is idle.
RF Connections
The DC voltage is isolated from the TV/video equipment by C7 and C13. 100pF ceramic disc capacitors are used, which give good coupling at UHF frequencies. The high frequencies are blocked by L1 and L2, which prevent the circuit from loading the signal.
There will inevitably be slight attenuation to the UHF signal; this has not been measured due to the author not having suitable equipment! No picture degradation occurred with the prototype, although some problems may be experienced in very poor reception areas. This would only occur on off-air signals, as the signal strength from the UHF output of a video recorder or satellite receiver is generally fairly high.
PCB Construction
The PCB overlays are shown in fig 2. There is nothing out of the ordinary about the PCB assembly - simply fit the components in the usual size order. D7 and D8 must be fitted on the solder side of the PCB, with their tops about 12mm above the PCB surface. D3 is mounted at the same height on the component side of the other PCB. D1 should be mounted at the full length of its leads, and then folded over so that the flat side lays against IC1.
Terminal pins should be used for the off-board connections. Those for SK1 and SK2 are inserted from the component side so that wires may be attached to the solder side.
There are four holes for terminal pins around IC1 and related components. These may be used to secure a screening can (made from tin plate) if the receiver is prone to interference. This was not necessary on the prototype.
Case Assembly
The infrared
receiver is housed in type MB2 plastic box, 101mm * 76mm * 39mm. A rectangular
window should be cut in one side, approx. 55mm * 24mm, positioned 12mm from
the left end. Remove any PCB mounting guides from this area. A piece
of red filter material is then fitted behind the cutout, and held in place with
superglue. If the filter has a non-reflective surface this should face
outwards. The PCB is positioned in the PCB mounting guides in the case,
immediately behind the window. The two coax sockets are fitted on the
opposite side of the case to the window, and require a 12.7mm (1/2") mounting
hole.The infrared transmitter is housed in a type MB3 plastic box, 118mm * 98mm * 45mm. A similar rectangular window is made in one side, 40mm * 26mm. The red filter material is again held in place with superglue. The PCB is fitted in the guides behind the window. The other side of the case is drilled to accommodate the two coax sockets and the mains cable entry. The latter must be fitted with a suitable cable clamp.
The transformer is mounted in the base of the case with M3 countersunk screws and nuts. An additional hole is necessary to mount a piece of choc-block connector, which is used to connect the mains cable to the flying leads from the transformer.
Cutting tidy rectangular holes in plastic cases is not easy! I drilled a hole in each corner and then cut out the remainder with a fretsaw. The hole was then filed to the correct size. Do not rush this section if you want to achieve a tidy job.
If the windows are cut to the sizes suggested you will be able to use one piece of Maplin red filter material for both cases. Contrary to the information in the Maplin/MPS catalogue, this material cannot be cut with scissors as it will crack. Use a junior hacksaw.
RS supply polarised red filter material which is darker and can be cut with scissors. This gives a more pleasing appearance than the Maplin material, but may be more difficult to obtain.
Interwiring
The interwiring is very straight-forward. The transformer secondary wires are connected to the veropins in the T1 position on the PCB, with the black wire to the centre pin and the two red wires (either way round) to the other two. The brown and blue primary wires are connected to the incoming mains cable with a 2-way piece of choc-block connector mounted in the case. Brown to brown, and blue to blue.
The coax sockets are connected to the relevant pins on the pcb using suitable coaxial cable. Cheap audio cable was used in the prototype and worked successfully, but the type specified in the parts list would be more suitable. The outer of each socket is connected via the screen of the cable to the relevant pin closest to the end of the PCB, while the core of the cable is used to connect the centre pin of the socket to the other pin on the PCB. Be sure to mark the sockets "LINK", "TV" and "VIDEO", in accordance with the circuit diagram.
Testing
No setting up is required, testing merely involves connecting the two sections and seeing if they work! When testing, ensure that the light from the transmitter does not shine directly on the receiver, or feedback may cause odd results.
Connect the two sections with a good length coaxial aerial cable between the "LINK" sockets (SK1 and SK3). Alternatively a length of two-core cable may be used; this may be soldered directly to the pins on the PCB's for convenience.
Connect the transmitter section to the mains and position it such that it is pointing at a video recorder, from about 2 metres away. Take the receiver and the video's remote control into another room, and try using the remote control about 2 metres from the receiver.
When the remote control is operated, the red LEDs on the receiver and transmitter should flash. If the channel change buttons are operated, the corresponding changes should be heard from the TV in the other room.
Fault Finding
If the unit does not work, there are a few points to check before embarking on a full faultfinding procedure.
First check the power supply voltages. There should be about 18V across C6 and C7, and around 12V at SK2. The power supply rail in the receiver (across D2) should be 5V.
Check that the LEDs are the right way round. The details in catalogues and data sheets can be confusing when it comes to identifying the polarity of LEDs, and different manufacturers use different arrangements. The PCB overlay is correct for the devices supplied by Maplin.
Check the aerial fly-leads for continuity, and short circuits. One of the two purchased by the author for these units was found to be open-circuit on the centre core!
If all this checks out, it's down to good old fashioned fault finding procedures. The circuit is not complicated so this should not take too long.
Installation and Use
In the interests of safety, all equipment should be disconnected from the mains before making any connections. However this is not essential if your video recorder is one of the older types that forgets the time if it is disconnected even momentarily from the mains!
The receiver should be positioned near the television, in clear sight of the normal viewing positions. Unplug the aerial cable from the TV, and connect it to the "LINK" socket of this unit. Using a standard aerial fly-lead, connect the "TV" socket on this unit to the aerial socket on the TV.
The transmitter positioning is more involved, and is left to the ingenuity of the individual constructor. The unit needs to be located so that the infrared output reaches the front of the equipment to be controlled. In addition the cables need to reach (or be extended), and the installation should look tidy if peace is to be maintained! The prototype was placed on a cabinet on an adjacent wall, and although the infrared reached the equipment from an angle of about 45 degrees no problems were experienced.
It may be easier in some cases to mount the infrared LED remotely, and link it to the electronics with a length of thin two core cable. Two or three LEDs could be wired in series, and placed near the receivers on the equipment to be controlled.
Some constructors may wish to try bouncing the infrared off a wall mirror at the opposite side of the room, although the distances involved may be too great.
There will presumably already be a Y-splitter connected to the output of the video recorder, with its outputs connected to the local and remote TV sets. Unplug the lead to the remote TV, and connect it to the "LINK" socket on the infrared transmitter unit. Connect the "VIDEO" socket on the infrared transmitter to the splitter.
Note that the infrared transmitter and infrared receiver must be at opposite ends of the link cable. There must be no splitters, attenuators, amplifiers, filters or other equipment between the two units, since these will block or load the DC path.
Finally connect the system to the mains (via a 3-Amp fuse) and test it.
If the LED on the receiver remains lit, it is picking up interference from something, try moving it further away from the TV set or other electronic equipment. This can be confirmed by switching off the TV or the suspected equipment.
As mentioned earlier, there is provision on the receiver PCB to mount a screening can around the sensitive preamplifier circuit. This may be necessary if interference is a problem.
The receiver should respond from a distance of at least five metres, if the remote control is aimed reasonably accurately. The range will vary with different makes of remote control. If the signal is weak the LED may still flash, but the remote equipment will fail to respond, this is caused by the receiver picking up only part of the signal. Try moving closer or putting new batteries in the remote control.
The transmitter should control the equipment from a distance of three or four metres, although this will drop off as the angle increases. Again this will vary with different equipment.
There should be no reduction in picture quality with this system installed. In areas of very poor reception it may be preferable to install a separate cable for this system, thin two-core cable (used for doorbells and speakers) is ideal. The "LINK" coaxial sockets may then be replaced with something suitable for the cable used, and the other coax sockets omitted.
Hopefully this unit will allow you to be even more lazy, just don't forget to take the remote controls with you! Happy viewing.
Parts List
Resistors (All 1% 0.6W)
1 R1 100R
3 R2,R4,R12 4K7
2 R3,R13 22K
3 R5,R10,R15 1K0
2 R6,R7 220R
1 R8 47R
1 R9 4R7
1 R11 330R
1 R14 470R
Capacitors
1 C1 47u 16V
1 C2 2u2 63V
1 C3 1n0
2 C4,C12 10n
3 C5,C8,C9 100n
1 C6 470u 10V
2 C7,C13 100p
1 C10 470u 25V
1 C11 2200u 25V
Inductors
2 L1,L2 10uH
Semiconductors
1 IC1 TBA2800
2 TR1,TR5 BC558
2 TR2,TR4 BC548
1 TR3 ZTX650
1 D1 IR Photodiode
1 D2 5V1 500mW Zener
2 D3,D7 RED LED 5mm
2 D4,D9 1N4148
2 D5,D6 1N4002
1 D8 IR LED 5mm
Miscellaneous
1 T1 12V 100mA Xformer
4 SK1-SK4 Coax Socket
1 Case MB2
1 Case MB3
1 Red Filter
1 pk M3 10mm Panhd Scrw
1 pk M3 Nuts
1 pk Veropins
1 metre Coax cable
2 metres 2 core 3A cable
1 Cable Clip
1 13A Plug
1 2A 25mm fuse
1 pair PCBs
As req'd Coaxial fly leads