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The infrared transmitter is housed in a type MB3 plastic box, 118mm * 98mm * 45mm. A similar rectangular window is made in one end, 50mm * 14mm. The red filter material is again retained by the PCB and LEDs. The PCB is fitted behind the window, with the red LED in the centre of the window. The transformer and connectors may now be fitted as shown in the photograph. The hole for the power cable should be fitted with a grommet, and the cable secured. The transformer should ideally be fitted with nylon screws.

Cutting tidy rectangular holes in plastic cases is not easy! The author drilled a series of small holes around the outside, removed the centre section, and then filed the edges smooth - a time-consuming approach. If a fretsaw is available this could significantly reduce the amount of filing required. Do not rush this section if you want to achieve a tidy job.

The red filter material used in the prototype was obtained from RS, however any type should be suitable. Thin material (0.5 - 1.0mm) is ideal since this can be cut with a pair of scissors. Test that the material chosen does not block infrared, by holding it in front of a remote control, and checking that the range is not significantly reduced.

The units can then be wired up as shown in fig **. The two core mains flex is joined to the transformer leads with a choc-block connector. The secondary leads from the transformer connect to the terminals on the PCB as shown.

The screen connectors of the two coax connectors should be linked with a heavy piece of wire - the braid from a piece of screened cable is suitable. Connect the silver mica capacitor directly between the centre conductor pins.

The inductors consist of about five turns of enamelled wire, approx. 22 SWG. Cut four 5" lengths and wind the six turns round the shank of a 3mm drill bit, towards one end of the wire. Connect the inductor wires as shown in fig **, with the coil towards the coax sockets.

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 reach the receiver, or feedback will cause odd results.

Connect the two sections with a good length coaxial aerial cable between the Link sockets. 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 15V at SK2. The power supply rail in the receiver should be about 13V, and the voltage on pin six of IC1 should be half this.

Check the polarity of D1, this device is used in reverse biased mode, and should have around 6V across it when the unit in inactive. Measure the voltage on pin six of IC2, if the voltage is around one volt or less then D1 is probably backwards. If the input of IC2 is touched with a finger the red LEDs should light or flicker. Make sure the infrared sensitive face of D1 is towards the window - with the half-round types available from Maplin (TIL100 equivalent) this the curved side.

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. However, there is a foolproof method. Look at the innards of the LED from one side, the larger piece of metal is the cathode and the smaller bit is the anode. The author always uses this technique and it hasn't failed him yet!