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In electrical controls, a stepping switch (also called a uniselector) is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. The major use for these devices was in early automatic telephone exchanges (commonly called Strowger exchanges) to route telephone calls. Stepping switches were invented by Almon Strowger in 1888.
Basic operation of a stepping switch
A basic stepping switch has a single input terminal (the stepping terminal) and multiple output terminals. Connection from the input terminal to the outputs is controlled by an internal rotary contact, or wiper, which can rotate, rather like the hand on a clock, so as to connect the input terminal (electrically connected to its central axis) to whichever output terminal it is currently pointing at. The position of the wiper is controlled by an electromagnet. Each time an electric pulse is received at the stepping terminal, the electromagnet is activated, causing the rotary contact to advance one position, and connect the input terminal to the next output.
In most applications, such as telephony, it is desirable to be able to return the rotary contact to a "home" position. Some stepping switches would rotate continuously back to the "home" position as soon as they reached the last position, while others had a separate "reset" coil and a return spring.
More complex switch configurations
One development was a type of stepping switch which had several rows of contacts, with one rotating wiper per row. All the wipers were mechanically coupled so that, as one rotated, the others remained aligned with it. Hence, multiple inputs could be connected to multiple outputs, based on the receipt of a single set of pulses. In this configuration, the rotating contacts looked, in general, somewhat like the head support arms in a modern rigid ("hard") disk drive.
Separate sets of cam-operated contacts on some switches moved when the rotor was at its home position, so that all of the bank of selectable contacts could be used for other purposes.
Although not as common, there were bidirectional stepping switches with two coils that could rotate the moving contacts in either direction, one coil for each direction.
Two axis stepping switch
Slightly more complicated was the two axis stepping switch (or Strowger switch) which had two stepping coils, one to rotate the wiper and one to raise it up to successive banks of contacts. These were commonly used in telephone switching with ten banks of ten contacts. These were driven by the electrical pulses generated from a telephone dial: As one digit was dialled, the wiper would step up the banks, then, as the next digit was dialled, it would rotate.
Another variant of the two-axis switch was the Stromberg-Carlson X-Y Switch which was quite common in telephone exchanges in the western USA. It was a flat mechanism, and the moving contacts moved both sidewise, as well as toward and away from a typical observer.
Applications of stepper switches
As well as the decoding of pulses from telephones, stepping switches could be used for a variety of purposes, depending on how they were wired. By connecting several in series with the highest output of one going to the stepping contact of the next, a counter could be constructed. Or by feeding the stepping contact with an endless pulse train via a relay, and controlling the relay from the switch's own output, it can be made to automatically hunt for the first unpowered line (or powered, depending on whether the relay is normally open or normally closed). They could also be used as a demultiplexer, so that two input lines could control a number of output devices. One input line steps the switch until the correct device is selected, and the other then powers that device. Many other applications are possible.
Such switches were also used in a series of Japanese cypher machines during World War 2: Red, Coral, Jade, PURPLE (The names are those used US cryptographers and have nothing to do with any aspect of the cyphers — they are the colors of binders used to hold information about the cyphers as they were worked on.) and in the effort to break Enigma - the "Machine Gun" used to check possible breaks takes its name from the loud noise made by the uniselectors.
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"Stepping switch"