Bob Evans of Southland Boys High School asks :-

How does a TV remote work?

Terry Morris, of Otago Polytechnic and the author of the book "Control Technology for Schools", responded.

Pressing a button on a TV remote control makes it flash a coded beam of light lulses. The light is infra-red and is invisible to our eyes. The light comes from a special lamp at the front called an LED (light emitting diode). A dark window, transparent to infra-red light, often protects the LED.

The on-off flashes last for different times. A group of flashes makes a command to control the TV. Other commands use different groups of flashes. An IC ("silicon chip") inside the remote waits for button presses. When it gets one it identifies the button, looks up the code and then flashes the LED.

The TV's infra-red sensor is usually found behind a small dark window at the front. Cover it and the remote won't work! A modern TV is just a computer by another name; inside is a special computer chip called a "microcontroller". Microcontrollers control machines and equipment rather than PCs. The TV's microcontroller waits for signals from the infra-red sensor. When one arrives the microcontroller identifies the code and then carries out the command.

The remote must point at the TV because it is using light. Sometimes it still works if you bounce the light off a wall and onto the TV because infra-red light is reflected just like visible light. Just remember that things which look white or reflecting in visible light may be "black" or non-reflecting in infra-red or vice versa.

Different brands of TV often use different sets of codes and their remote control units only operate that brand. Many homes have several remote controls for different appliances. This can be inconvenient and so it is possible to buy remote control units fitted with their own microcontrollers capable of learning the codes of other remotes. In that way one remote control can do the job of several.

The first TV remotes were built in the early 1950's. They used bulky cables and weren't popular because people tripped over them. Cable remotes were replaced by systems using four photocells positioned at the screen corners. The TVs were controlled by shining a torch on the photocells; the system got rid of the cables and it worked OK in dim light but throwing open the curtains on a sunny day caused TVs to change channels, cut the sound or worse.

After the photocells and torches came ultrasound remotes which used high pitched sounds well above human hearing. A small handset made the sounds using mechanisms rather than electronics. Although much more reliable than the photocell system it wasn't perfect; unwanted high pitched sounds also made TVs misbehave. Anyone with a dog-whistle wasn't popular near the TV and the remote wasn't too popular with dogs!

Infra-red remote control is reliable but the system still isn't perfect. Homes may have many remote controls and it is very annoying when they get lost or when the wrong buttons are pressed. Manufacturers want to get rid of the remotes and replace them with speech control. TVs and other appliances will then be clever enough to "understand" spoken commands. Despite what Hollywood movies might suggest, however, making a computer recognise speech isn't easy and much is still to be done.

If you want to learn more about infra-red control, look at the Picaxe microcontroller chip ( now used by many New Zealand schools. It has special instructions for working with infra-red light. Why not build a Picaxe infra-red remote controller for a buggy?