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Infrared Transmitter/Power Supply
The variations in supply current to the receiver section cause a varying voltage drop across R20. This is converted to logic pulses by TR5 (BC558). C8, R16 and D8 cause short (8uS) pulses to be applied to the base of TR4. TR4 (BC548) and TR5 (ZTX650) are in a Darlington arrangement, and control 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 about 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. C7 acts as a reservoir for the LED current, and is charged when the LED is not lit via R13. The current to D7 is limited to about 1.5A by R15; a red LED (D6) is connected across this resistor 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 C16. The supply reaching the infrared receiver section will be about 13V. A 100mA transformer is adequate since the current consumption is only a few milliamps when the unit is idle.

RF Connections
The units are connected to the aerial cable as shown in the interwiring diagram.

The DC voltage is isolated from the TV/video equipment by C9 and C10. This is a silver mica component which gives good performance at UHF frequencies. The high frequencies are blocked by L1 to L4, which prevent the circuit loading the signal.

There is inevitably some slight loss 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 problems may be experienced in poor reception areas.

Construction
For convenience the PCB's for the two parts of the circuit are supplied in one piece. The first job is to cut this PCB in half. Fix the PCB to a bench or table with a small G-clamp, and cut along the dotted line using a hacksaw fitted with a fine blade. The four fixing holes in each section should now be drilled out to 3mm.

The PCB overlay is shown in fig **. There is nothing out of the ordinary about the PCB assembly - simply fit the components in the usual size order. The IC's may be fitted in sockets, but since they are low cost devices this is not really necessary. Terminal pins should be used for the off-board connections. Do not forget the two links on the receiver section.

The LED's and infrared photodiode are mounted in line with the edge of the PCB, about 10mm above the surface. This should be visible in the photographs. Check the height of the larger electrolytic capacitors, it may be necessary to lay these components on their sides or obtain small modern devices.

The general layout in the two cases can be seen in the photographs.

The prototype infrared receiver is housed in a plastic case 110mm * 61mm * 30mm, which was purchased from Tandy. However, cases of this size do not appear to be readily available by mail order. It would probably be better to obtain something larger, such as a type MB2, which is readily available and will match the transmitter case. A metal case may reduce the probability of interference from the TV set (this has not been tested), however this would be more difficult to machine.

The receiver case has a rectangular window in one end, approx. 30mm * 14mm. Remove any PCB mounting guides from this area. A piece of red filter material is then fitted behind the cut-out, in the prototype this was held in place by the PCB and LED. If the filter has a non-reflective surface this should face outwards. The PCB is positioned in the case immediately behind the window, and fitted with M3 screws, nuts and spacers. The two coax sockets are fitted on the opposite end of the case to the window.