Originally published by Paul Stenning in Electronics and Beyond (The Maplin Magazine), February 1997

• Diagrams and Schematics

Introduction
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.