Andy Honeybone switches on all the lights, switches on his portable typewriter, switches on inspiration, and tells you all about how switches switch

A switch, like good health, is very easy to take for granted. It's quite easy to walk into a room and use several without even thinking. A quick browse through a catalogue shows about 15 basic types and hundreds of different varieties. Despite this enormous choice, you can never get an exact replacement for a defunct item. During my days as a counter hand in an electronic component store, when presented with a handful of broken switch and asked if we had one, it was standard practice to say, 'no, all ours are in one piece'. Such are the depths of humour which retailing drives one to (testing?).

This article is rare in that it is an actual commission. Usually, I approach the Editor with a suggestion and he tells me why it's a bad idea. (I still say 'When is an Alpine Horn?' would have been a winner.) This time, his nibs comes on the blower and fronts up with this switch business — a thought put into his mind no doubt by the impervious qualities of the membrane keypads on a DX-7 to the ingress of promotional launch booze. Stifling a yawn, I loped off to research the subject and almost got enthusiastic. Using the switch as a vantage point it's possible to look at a whole lot of musicians' kit that would be difficult to cover in any other way within this series.

Baron Frankenstein would think of a switch as a pair of lengthy brass blades operated by a wooden handle. Of course, a shower of sparks would fly out when contact was made with the stationary conductor and it would always appear to be an act of superhuman effort to throw the switch — rather like the way that Manfred Mann used to grimace when turning the knobs on a Mini-Moog. To the musician, a switch might be the delicate gold-clad contacts beneath an electronic keyboard or the rocker switch, so aptly named, which directs power into a 200 watt valve amplifier head. In all cases, the switch makes or breaks a path along which current can flow.

The term 'switch' is some what general and before we progress too far it might be worth getting our terminology together. The simplest differentiation can be based on what happens when the actuating force (the finger) is removed. If the switch stays switched, it is said to be latching but if it releases (back to its unfingered position) it is said to be momentary. The two types each have their place; you would not want to hold a light switch on all evening and neither would you want a door bell to ring for extended periods. The rest of the description is based on the mechanics of the switch and this is fairly obvious. Toggle, paddle, rocker and push-button are very similar in the way that they couple the motion to short out the internal Contacts.

The other main group is the rotary types such as those used to select oscillator waveforms in the days before computers wormed their way into synthesisers. The advantage of rotaries is that any number of additional isolated switches can be coupled to the same shaft by including extra wafers. We number of separate switched paths is known as the number of poles and the choice of connections within each pole is known as me number of ways. A single way switch is therefore a simple on/off device; a two-way switch is one where a common connection may be switched to either of two terminals, for example, directing a signal to either the left or right channel. A double-pole, two-way switch that was double-throw (having a centre off position) can be used on an amplifier to reverse the polarity of the mains input. This common feature of American amps allows selection of the configuration giving the lowest hum. Two cross-straps are needed such that reversal is achieved on one side of the switch relative to the other. Two-way switches are often referred to as changeover types and non-rotary versions exist with a maximum of four poles. Rotary types have one further parameter, they may be specified to be make before break (MBB) types. This feature means that during switching, there is no period when the moving contact is left floating. MBB switches are highly desirable on audio attenuators where it would be disastrous should a burst of full power occur when making an adjustment.

There are many special-purpose switches which have no immediate musical application. Thumb wheel types are getting rather long in the tooth these days but banks of the things were the traditional way of entering numbers into electrical machines. The term comes from the rapid removal of the thumb nail when turning the outer spokes of the numbered wheels which were mounted sideways on. The output of such a switch was four bits of binary coded decimal. The mercury-filled tilt switch of pinball machine fame has been used in an alarm system for protecting guitar cases which have to be left unattended.

Perhaps the most popular at present are the touch and membrane types. The first touch switches to be incorporated on a commercial synth were as voice selectors on the Oberheim OB-1. They took the form of a row of upholstery tacks, the heads of which where dabbed by the human finger. The principle behind this no-moving part technology is much the same as when you touch the end of a jack lead plugged into an amplifier. Your body acts as an aerial which picks up all the stray mains fields around you and channels them down the wire for amplification. If instead of a loudspeaker there is a level detector at the end of the amplifier, you have a touch switch.

This method is not terribly reliable and other systems have a local oscillator which is coupled to detector circuitry when the digit is presented. It is necessary to provide some form of feedback to the operator to indicate that the switch has switched — something like a click or beep — or else far too much pressure can be applied. Touch switches suffer from their contacts becoming fouled up which results in unpredictable response. Some people seem to be able to activate them better than others, no doubt due to body capacitance and skin moisture content.

The slick facias possible with touch switches have been achieved with minimum undulation and far better reliability by membrane technology. The switches on a DX-7 are of this type and typically they consist of a dish of springy metal that can be inverted by finger pressure to short out underlying contacts. Positive feedback is inherent and because of the simplicity of construction, five million operations are quoted as the life expectancy. The assembly can be covered with a compliant panel and this excludes all dirt and grunk.

A derivative of the membrane type has a conductive rubber contact which is carried under a dimple moulded from silicone rubber. When placed on a printed circuit board bearing suitable patterns, a switch is formed which is extremely cheap. The feel of this switch has been likened to 'dead fish' and so a plastic button can be 'floated' on the top. Both touch switches and membrane types are mechanically momentary but it is their associated electronics which gives them their overall characteristic. Neither exhibit a true short circuit when closed — the rubber type may only fall to 100 ohms.

To close, two other switches: the noise gate and the earth leakage contact breaker. An electronic switch which opens in the presence of a signal is termed a noise gate. Its uses vary from isolating inputs from a multi miked drum kit to cutting background noise and creating altered ambience effects. The switch takes the form of either a field effect transistor, a bipolar transistor or an analogue transmission gate. These devices are just as much switches as a mechanical device but can be thrown by electronic means. More sophisticated noise gates have voltage controlled amplifiers within and fade the noise out rather than cutting it dead.

The earth leakage circuit breaker (ECLB) is a device which could save your life. It is a switch which holds on all the time that the current in the live mains wire is balanced by the current flowing in the neutral. If a short to ground occurs — that is through a body — the balance is thrown and the unit disconnects the mains. Such a device will trip at about 30 milliamps, enough to cause a tingle but not enough to kill. They are available as units with a built-in 13 amp socket and trailing mains plug so fitting is no problem — certainly no excuse for an accident switching off your career for good.