Clocks

This page is a stub. Eventually it will cover various types of clock.

Summary

Clocks have seen many changes with evolving technology.

Safety

You may miss your bus if your clock is wrong. This could really spoil your day.

Basic Principles

All clocks can be considered to be composed of two parts:

• Some kind of time standard for measuring the passage of time, and
• Some kind of display for displaying the measured time.

In principle you can mix and match the two, so for the purposes of explaining basic principles it's worth considering them separately.

Time Standards

Mechanical

Pendulums and balance wheels are the oldest accurate means of measuring the passage of time. They were refined in the 18th Century most notably by John Harrison, spurred by the need for an accurate clock for navigation at sea. All these clocks rely on the simple fact that if a mass is subject to a force tending to restore it to an equilbrium, with the magnitude of the force being in direct proportion to the distance from that equilibrium position, the mass will execute "Simple Harmonic Motion". This is characterised by an oscillation or vibration at a well defined frequency, independant of the amplitude of the motion. This is why a wind-up clock doesn't run slower as it runs down, like a wind-up toy car would, for example.

In practical clocks, an escapement is needed to give the pendulum or balance wheel the regular kicks it needs to keep it indefinitely in motion. This can be powered by a spring or in the case of a grandfather clock, by weights. Occasionally in more modern clocks you might find an electrical mechanism.

Very rarely you might find a clock which uses a tuning fork kept vibrating electronically as the time standard instead of a pendulum or balance wheel.

Quartz crystals

These in fact also depend on a mechanical vibration, but sustained electronically and at a much higher frequency.

Quartz is an example of a piezoelectric material, that is, an electical charge appears on opposite faces if you stress it, and conversely, applying a voltage will cause it to deform slightly. Using this property, a piece of quartz is carefully cut and ground so as to "ring" at a specific resonant frequency and is kept in oscillation electronically.

Quartz crystals used in clocks nearly always have a resonant frequency of 32,768Hz. This might seem an odd number until you realise that halving it 15 times successively gives a 1 second tick. This is easily done with simple electronic circuits. An advantage of such a high frequency is that you just have to count the oscillations in 1 second and check there are exactly 32,768 (not even just one more or one less) to assure the accuracy of your clock to one second in 9 hours! If a clock ticks just a few times a second, you would have to count the ticks for a much longer time to achieve the same accuracy. Fine adjustment is possible by connecting a variable capacitor in the circuit with the crystal.

Quartz crystals provide a more accurate and more stable time source than a mechanical clock but nevertheless they are somewhat temperature dependant. Those used in cheap clocks typically have an accuracy of a few seconds per day. Additionally, quartz crystals are subject to ageing, causing a small additional drift over a number of years, and are also affected by the heat of the soldering process at the time of manufacture

In a Temperature Compensated Crystal Oscillator (TCXO) the control IC contains a temperature sensor and the crystal is incorporated in the same package. This allows the IC to compensate for temperature variations by adjusting the capacitance included in the crystal oscillator. By this means, an accuracy of 10 seconds per year is achievable.

MEMS Resonators

A resonator can also be constructed as a MEMS (MicroElectroMechanical System) device in which a microscopic mechanical resonator and the required control circuitry are all fabricated on a single silicon chip. This can give an accuracy similar to a TCXO, but it is inherently much less temperature dependant, less subject to ageing and the effects of soldering, and at the same time much more rugged.

Atomic clocks

These are the most accurate clocks available, the best typically gaining or losing no more than the equivalent of 1 second in 30 million years, but they are very expensive and usually very bulky. Chip-scale versions are now available but these are still by no means cheap.

Atomic clocks depend not on a mechanical vibration but on an atomic resonance, most usually in caesium or rubidium atoms. In fact the second is now officially defined as 9,192,631,770 oscillations in a caesioum-133 atom. National and international time standards are based on the averages of a number of atomic clocks, and it is on these that all the remaining time standards are based.

Rubidium frequency standards (not quite as accurate as caesium ones) are widely used in cell towers. In fact, if you wanted your very own atomic clock there are second hand ones from decommissioned cell towers going on eBay for only a few hundred pounds!

Radio Time Signals

In the UK, the National Physical Laboratory's time standard is based on three atomic clocks, and is broadcast as the "MSF Signal". This can be received across much of northern and western Europe. Numerous other radio time signals are available in different parts of the world.

Domestic clocks which automatically synchronise with one of these radio broadcasts are widely available, sold as "radio controlled clocks", or even "atomic clocks" though they are actually using someone else's atomic clock!

AC Mains

The AC mains supply has a nominal frequency of 50Hz (or 60Hz in some regions). Although the frequency can vary slightly, the total number of cycles in a day is very carefully controlled. The reason for this is that if demand exceeds supply, all the generators naturally tend to slow down under the load, so reducing the frequency, or speed up in the case of over-supply. Hence the frequency is used as a vital tool in matching supply to demand in the National Grid.

A useful consequence is that the mains supply can be used as a time standard with guaranteed long term accuracy, though like several others, it's in reality a 2nd hand atomic clock.

GPS

Satellite navigation depends on a constellation of satellites each with its own on-board atomic clock and transmitting a signal containing encoded time information. Consequently not only can you get your geographical position by receiving these signals, but also an extremely accurate time.

Internet Time Sources

Your computer, smartphone or tablet contains a quartz crystal from which it determines the time on a continuous basis, but this is regularly synchronised with time sources available on the Internet. NTP (Network Time Protocol) is the means by which any computer on the Internet can request the time from a chosen time server and estimate the adjustment required to account for the transmission delay. The necessary correction to the system clock is then made gradually so as to avoid a sudden jump in the time, especially a jump backwards which might be very confusing to some applications.

NTP time sources are each given a "Stratum" classification. A Stratum 0 reference is an actual atomic clock or equally authoritative time source. A Stratum 1 device periodically synchronises against a Stratum 0 reference. This might be a rubidium clock or a server accessble on the Internet. An organisation might have its own Stratum 2 server which periodically queries a Stratum 1 server, and in turn is used as a reference by Stratum 3 devices, which might be employees' PCs.

Time Displays

Analogue Displays

We all learned at an early age to tell the time from an analogue clock face with a minute and an hour hand, and for centuries, this has been the standard method of displaying the time. Imaginative novelty variations on the idea are sometimes seen.

Digital Displays

Electromechanical digital displays have been around for many years, the commonest form being the flip-down display. A series of cards flip down one after another, each card containing the top half of a digit on one side and the bottom half on the reverse. These clocks are now often collector's items or cherished heirlooms.

Nixie tubes were used as an early form of digital display. A Nixie tube consists of a glass envelope containing neon gas and 10 digits, each formed out of wire. Any one of these can be illuminated by applying a voltage to it, causing it to light up with the characteristic orange neon glow. Special driver circuits are required as Nixie tubes need around 150V to work. Nowadays these clocks are purely a novelty item, popular amongst the maker community.

Modern digital clocks generally use a familiar 7-segment display. The entire display may be formed as an LCD panel, often including additional symbols for AM/PM or an alarm indication, or each segment may be lit by an LED. LCDs and LEDs have the advantage of working on similar low voltages to the logic circuits driving them.

Vacuum flourescent displays (VFDs) are most often seen on older devices such as video recorders. The flourescent segments are housed in a glass envelope and are made to glow by a stream of electrons, often with a blueish green colour. They tend to use higher voltages than most logic circuits but not as high as Nixie tubes.

Clockwork clocks

These are the oldest type of clock and are purely mechanical. After many years of service they often stop working because of wear in the bearings and gears, and cheap clocks often fail much sooner. Repairing these is a highly skilled operation, if possible at all.

Fault-finding and repair

As with other types of clock with brass bearings and gears, you must only use oil specially intended for clocks. Such oils leave no residue if they gradually evaporate, do not disolve any lacquer that might be applied to the brass, and tend to stay within the bearing.

Electrically-assisted clocks

These have a pendulum or a balance wheel like a clockwork clock, but it's kept in motion electrically instead of by a spring.

Mains synchronous clocks

These were very popular as accurate mantlepiece and wall clocks before the advent of quartz movements. A synchronous motor rotates at the mains frequency and a gear chain reduces this to drive the second, minute and hour hands.

Fault-finding and repair

First of all, it must be said that there are serious safety issues with these clocks. For a start, the insulation is likely to be in a poor state, and to make matters worse, they have exposed metal parts and rarely have an eath connection. This means there is a significat risk of electrocution and possibly also of fire. However, this doesn't mean they can't be repaired or restored.

Often, you will find the coil in the motor is open circuit or burnt out (or both). This coil is driven directly by the mains and creates the oscillating magnetic field which turns the motor spindle. Whilst it may be possible to rewind this coil it's likely to have many thousands of turns and the wire will be so thin that you'd be unlikely to complete the task without it breaking.

A good alternative is to rewind it with fewer turns of somewhat thicker wire and drive it from a low voltage transformer, ideally an AC wall cube type. This has the added benefit of eliminating the risk of electric shock and greatly reducing the fire risk. How to do this is covered in detail at sound.whsites.net/clocks/ocm.html.

Quartz clocks

These rely on the vibrations of a crystal of quartz for their time keeping, rather than a balance wheel or pendulum.

Fault-finding and repair

Sometimes the setting buttons become unreliable. If they are the standard miniature tactile type you may be able to replace them, but often they just consist of a rubber button with a conductive layer underneath which makes the contact between adjacent circuit board traces. You may be able to improve it by cleaning the contacts or by rubbing a 2B (or softer) pencil on the conductive layer, or applying conductive paint such as Bare Conductive. Yet another type of button consists of a piece of domed metal which clicks like the safety button on the lid of a glass jar to make contact with a circuit board trace.

As with many battery operated devices, dead batteries may leak and corrode the contacts, which will need thoroughly cleaning.

If you remove the hands (these just pull off), refit them both precisely on the 12 so that the hour hand will always point at an hour when the minute hand reaches 12.

Quartz Watches

These run off a coin cell which typically lasts around a year. More modern ones may have a lithium coin cell lasting around 3 years.

Fault-finding and repair

You can save money by changing the battery yourself, but you may need special tools.

The back will be either screw-on or snap on. A screw-on back will have several notches into which you can engage a tool to twist it off. You can screw it back on in the same way. Snap-on backs can be harder both to remove and replace. There will be an indentation at some point around the circumference into which you can insert a knife of removal tool. Considerable force may be required so great care is needed especially if you are using a sharp knife, in case it slips. Replacing it also often requires considerable force. If you can't do it between fingers and thumbs you will need a watch press. This comes with several dies of different sizes designed to apply the force to the watch body rather than the glass, which you would be very likely to smash.

Having got inside you will probably have to loosen a very small screw in order swing a contact out of the way so as to remove and replace the battery. Only loosen it as much as you need to as it will be fiddly to replace if it comes right out.

You may be able to clean up the battery contacts if they are corroded, and provided the leaking electrolyte which caused the corrosion hasn't caused any other damage. Any other faults such as water damage may not be feasible to repair.

Radio-controlled clocks

These generally have a quartz clock mechanism for back-up timekeeping, but get an accurate time reference from a radio signal, such as MSF Rugby, against which they synchronise regularly.

The self-setting mechanism relies on the clock being able to detect when the hands are in the 12:00 position. This relies on holes in the minute and hour gears which line up at 12 o'clock. An LED and photodetector sense this position. Some clocks have a slightly more complicated system allowing the clock to run fast until maybe 11:45 and then more slowly so as not to miss 12:00 precisely, or they maybe able to recognise several positions of the hands, avoiding the need to wind forward to 12 o'clock in every case. All these use the same kind of mechanism, though understanding just how the clock recognises different alignments of the holes may not be easy.

Fault-finding and repair

Sometimes the problem with these is quite simply an unsuitable battery. Make sure it's an alkaline type, not a cheap zinc one, or a rechargeable.

The other simple problem is just that they may be placed somewhere where the radio signal is inadequate. Try leaving the clock on a windowsill.

The same problems with push buttons occur as with quartz clocks.

There is probably little you can do about mechanical problems. Should you remove the gears it's likely that they will need to be reassembled in the correct orientation for the setting mechanism to work. If you examine them carefully you may find that the critical ones each have a pin hole which needs to be lined up one with another or with a pin hole in the plastic case. Inserting a pin through these holes during reassembly will ensure the correct orientation. This is likely to correspond to the 12:00 position of the hands.

Flip-down clocks

These are the earliest widely available form of digital clock. The hours and minutes and possibly the day of the week and day of the month are displayed on flip-down cards.

7 segment display clocks

These may use a quartz crystal, radio signal or the 50Hz mains supply for the time standard.

External links

• Watch and Clock Movements at Electropedia gives easily understood explanations of everything from pendulum clocks to atomic clocks.