Switch on

7th March 2003 at 00:00
There are loads of ways to turn machines on and off and send electronic signals. John Stringer looks at how to explore and explain them

You are woken by the sound of your alarm clock. Staggering to the bathroom, you tug on the light. The electric toothbrush buzzes as you put it in your mouth. You blow your hair dry and go downstairs to put on the kettle, pausing only to zap the television into life. The fridge lights up as you open the door. The phone rings and you lift the receiver, tapping an appointment into your electronic diary. When you leave the house, the security lamp casts a path of light in front of you. The car coughs and starts. Wogan is on the radio, and the rear screen quickly demists. You indicate to leave the roadside and start the drive to work.

By pushing, pulling, turning, flicking, squeezing or walking past, you have operated a variety of different switches and sent a myriad of electric signals. These switches - and many more like them - have one thing in common. They complete or cut the flow of electricity. We use them without thinking.

Making use of free air

Switches simply "make" or "break" an electric circuit. You could get the same effect by putting a spanner across your fuse box - lights on - or an axe through your mains cable - lights off. But this physical and wholesale approach to lighting up your life could have long-term - even fatal - results. We often need to be insulated from the electric current.

Air is an insulator. Unless the current is massive, air will prevent it from jumping to us. All that switches do is to introduce air into the circuit. That's what happens whenever you break the complete circuit of electric conductors - the wires and metal bits that let the electricity flow easily.

The devices we think of as switches are simply insulating boxes that keep your fingers away from a shocking experience. In theory, you could light a low-voltage torch by simply holding a couple of wires together. But stepping from the bath and inserting your finger into a high voltage wall socket is likely to make your beneficiaries wealthy overnight. The human body may not be the greatest conductor, but a coating of water reduces its resistance to the flow of electricity. Coming into contact with electricity when you are wet is likely to produce some muscular activity that, especially if it involves your heart muscle, will probably be fatal. That's why you are additionally isolated from most bathroom switches by a length of string.

Don't just stand there

You can divide most switches into those that need you and those that don't.

Doorbells, keyboards and power drills need you to keep squeezing to complete the circuit. Televisions and bedside lights are happy to get a moment of your attention and then to keep the circuit open or closed for you. This makes a lot of sense. Imagine a doorbell that continued to ring long after you had opened the door to your guest, or a kitchen light that would only work while you kept pressing it. Many potentially dangerous devices, such as hedge trimmers and chain saws, demand our full attention.

With push-button switches, some children don't recognise their own part in the circuit and assume that the button on the door actually chimes.

"I Puts the Lightie On"

Tommy Steele had a hit with this irritating title in the 1960s. His novelty record proposed that a small and presumably very cold man lived in your fridge. By opening the door, you brought him to life, and he lit the fridge for your convenience. But the truth is rather less exciting. Many home appliances have "push-to-break" switches. As long as they are pressed - by you or by the closed door - they break a circuit and stop the electricity flowing. But take your finger off, or open the door, and the circuit is completed and the fridge - or Del Boy's cocktail cabinet - lights up. But sometimes you don't even have to touch anything to make or break a circuitI Who opened the door?

It can be disturbing to enter a public toilet only to find the lights seem to be switched on for you by an invisible hand. Most of us have got used to hand-dryers that come on when we shake our wet fingers underneath, but it can still come as a shock when the urinal flushes as you walk away.

Proximity switches such as these control many devices, such as movement or heat sensors for security lights, but they can be easily fooled. A tabby cat in the garden is treated like a cat burglar and caught in a blaze of light - and steam from an air vent can also set off a response. (The computer mouse is also a kind of movement detector - see page 22).

Sometimes when you walk into a shop a door opens as if by magic in front of you. You've not consciously pressed a switch, but in a way you have. You've walked between a source of focused light (possibly a laser beam) and a light sensor. When you do this, you briefly block the path of the beam. The sensor registers the difference in light levels and sends a signal to the control box which opens the door. (For more about lasers, see the graphic on CD-Roms , right.) No pressure

Some switches operate without any human contact. They detect a change in the environment and respond to it automatically. A thermostat switches on the heating when the room temperature drops. You can modify its behaviour by turning it up or down to make the room warmer or colder - but you can't usurp its purpose in life, to respond to a drop in temperature by switching on more heat. Similar switches respond to low light, which is why the streetlights may come on during a grey day.

It's time for a cup of tea. Put the kettle on - it will turn itself off.

Mouse control

There are several switches in a computer mouse. The main, hand-operated switches - the left and right buttons - trigger a chip in the mouse to send signals to the computer. Movements of the mouse roll the ball, which turns a pair of rollers at right angles to each other. These are joined to slotted wheels. Light from a tiny light source - a light-emitting diode (LED) - shines through the slots, and light-sensitive switches detect the onoff signals, using them to signal the movement of the mouse to the computer.


A computer is not always controlled by a mouse. A computer game joystick has a pair of sliders in its base, which are electrically changed by tweaking the stick. Buttons and triggers control on-screen jumps and shots.

Touch-sensitive switches make it possible to run your finger across the rectangular pad of a laptop computer, rather than pushing a mouse round the desk. Touching the pad changes the electric charge in the pad's circuits, producing variations in the signal. The cursor on the screen follows your finger. Ever tried to race it? You'll have to be pretty quick with your finger - these switches can follow speeds of up to a metre a second. Even neater than the plastic pad are computer screens you can touch. The screen glass contains a weak electrostatic field within it that is disturbed by finger contact. A layer beneath the glass detects these charges, and they are recorded by a base layer of glass coated with an electrical conductor, which transmits the movements to the processor. The processor is constantly calculating the co-ordinates of your fingertip, but it can't yet wipe sticky finger marks off the screen. Perhaps, after all, you need to keep your hands off altogether, and leave it all to the machine.

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