Category How does it work?

When your clock radio starts playing music first thing in the morning, or the oven automatically comes on to cook a meal, the switch has probably been operated by a digital clock.

At the heart of the switch is a quartz crystal which vibrates at a fixed frequency when connected to a source of electrical power – battery or mains. The vibrations produce regular electrical pulses, which travels through circuits in a microchip to operate the digits on the clock.

The switch also has a memory, in the form of a microprocessor, which stores the time when the radio, oven or central heating system has to be turned on. The microprocessor constantly compares the stored time with the real time as measured by the clock.

When the turning on time comes, it sets off a low-voltage electronic signal. This signal is amplified by a transistor circuit and flows through a relay, an electronic device in which a small current causes a metal contact to move, switching on the main current.

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Car tyres are not just cushions for the wheels. They are there to give the car a good grip on slippery roads, and stop it sliding about when braking or cornering.

The tread pattern running all round the tyre has thin cuts (known as sipes) in the rubber to sponge up surface water, and zigzag channels to pump the water out behind as the car rolls forward. On a wet road, a tyre has to move more than 1 gallon (5 litres) of water a second to give an adequate grip.

On a perfectly dry road, the treads are not needed. A smooth tyre gives the greatest possible area of contact with the road. But if the smooth tyres are used in wet weather, the film of water on the road builds up in front of them and underneath them and actually lifts them and off the road surface – this is known as aquaplaning. When aquaplaning occurs, the driver loses control.

Most cars have to function in all weathers, so must have tyre treads, but racing cars make comparatively few outings a year. If the track is dry, they run on smooth tyres, called slicks, to get the best grip on the roads. The extra wide tyres and wheels give more grip that the average cars. In wet weather, however, the slicks have to be changed for treaded tyres.

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When you are travelling in a car, you and the car are moving at the same speed. If the car stops abruptly, your body keeps moving forward. This is an illustration of inertia – the tendency of a moving object to keep moving, or of a stationary object to remain at rest.

An inertia-reel seat belt works on the same principle. Its mechanism includes a pendulum, which hangs vertically under ordinary driving conditions. But if the car stops abruptly it swings forward, and a locking lever resting on the pendulum is released. The lever engages a toothed ratchet that locks the shaft round which the belt is wound. The locked seat belt then prevents your body from being flung forward.

When you fasten a seat belt, it winds out from the reel against slight tension from a spring. This keeps it taut during normal travelling, but allows enough free movement for a driver to reach forward as necessary. But if you tug abruptly on the belt while winding it out, the locking mechanism will engage and stop the action of the spring. Slackening the belt releases the spring and the locking lever.

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The exquisite workmanship of the traditional mechanical wristwatch has given way to the magic of the microchip. In the quartz watch, a vibrating crystal has taken over the role of timekeeper from the traditional finely tuned balance wheel ad hairspring. Minute electronic circuits control its operations.

A quartz crystal vibrates at an unvarying rate when an electric current is passed through it. The man-made quartz crystals used in watches are usually designed to vibrate 32,768 times a second when stimulated by the current from a battery. These vibrations produce electric pulses, and as the pulses travel through the electronic circuits of the microchip, their rate is successively halved in a series of 15 steps. The result is one pulse per second. Each one-second pulse triggers the chip to send signals to the digital display to advance the numerals one second.

The chip also uses the pulse as a base for other counting circuits, such as those that display hour and date, and to control the alarm signal.

Many modern quartz watches display the time in digits on a liquid crystal display (LCD). The liquid crystals are sandwiched between a reflective bottom layer and a top layer of polarised glass, and transparent electrical conductors separate them into segments. Each digit is formed from segments – up to seven are normally used, all seven being used for figure 8.

The liquid crystals rearrange their molecules according to their electrical state. Where the conductors carry no charge, light through the sandwich is reflected out again, so the display is blank. When the conductors are charged by an electric pulse, the molecules in the affected segments realign and twist the light away from the reflective surface, so the segments appear dark.

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In the split second between the pressing of the shuttle release and the opening of the shutter, an automatic camera measures the distance between the lens and the subject and positions the lens to give sharp focus.

Most compact cameras have a tiny electric motor driving a transmitter that emits a beam of infrared light. The transmitter is linked to the lens, which moves in or out as the beam scans – focusing from near to far. The beam reflects back from objects to the camera, and a sensor monitors its signals and stops the transmitter when the strongest signal shows that the lens is in focus. This automatically triggers the shutter.

Some instant cameras have ultrasonic focusing similar to the echolocation scanning system bats use to navigate. A gold-plated disc (the transducer) sends out ‘chirps’ too high to be heard by human ears, each lasting 1/100th of a second. The disc receives the chirp echoes from the subject, and a built-in microcomputer measures the time each chirp takes to go out and come back. From this it calculates the distance to the subject.

SLR (single-lens reflex) cameras with an auto-focus use what is known as an electronic phase detection system. In this, light entering through the lens is separated into two images. A sensor measures the distance between the two images, which are a specific distance apart when the lens is in focus. If the distance is not correct, the sensor causes a motor to move the lens.

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