Originally published by Paul Stenning in Electronics in Action, January 1994

• Diagrams and Schematics

Introduction
The author, like many people, has 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 one 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 that lives upstairs by the TV, and the infrared transmitter that lives downstairs and points at the equipment to be controlled.

The Works
For now, assume that the two sections of the circuit are connected directly (SK1 joined to SK2). The connections to the aerial cable will be described later.

Infrared Receiver
The infrared signal from the remote control is received by D1 (TIL100), which is connected in reversed bias mode. The reverse leakage of this device increased when it is exposed to infrared, causing a variation in the voltage on pin three of IC2.

IC2 (LF351) buffers the signal, which then passes to IC3 via a high pass filter. This filter removes low frequency variations caused by mains lighting. IC3 (LF351) has a gain of about two thousand, which increases the minuscule signal to something usable.

The unit can receive the signals from a remote control up to about three metres away, which will generally be adequate. If necessary the sensitivity can be increased by changing the value of R5 to 10M, however this will not make a huge difference and may make the unit more susceptible to noise.

C4 is not normally required, but may need to be fitted if the unit is prone to RF interference. A value between 2.2pF and 10pF would be suitable here, although this will decrease the sensitivity.

Note that the unit may pick up interference from the line output circuit of some television receivers. Since this is at 16KHz, fitting C4 will have no effect. Filtering out this interference would require a fairly steep filter, which would defeat the objective of trying to produce a simple low cost unit. If the unit is moved one metre away from the TV set the problem disappears.

The output from IC3 is converted to a squared logic signal by IC4 (CA3140), which is configured as a comparator. The output of this 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 C1 ensure that the power supplies to the IC's does not vary significantly when the LED is lit. IC1 produces a power rail at half the main supply level.

The author is aware that infrared receiver IC's are available which will achieve similar results to the above circuit. However these only appear to be available from the larger distributors, and are not likely to be found in the average constructor's junk box. Besides, there is more satisfaction in designing from scratch, rather than just lifting a circuit from a data sheet.