In electrical controls, a stepping switch (also called a uniselector; see Strowger switch, below) is an electromechanical device used, most prominently, in early automatic telephone exchanges to route calls. A basic stepping switch would have (usually) two stepping terminals, one input terminal and some number of output terminals.

Each time an electric pulse was received at the stepping terminal, that power went to an electromagnet which drove an internal rotary contact (called the wiper, connected to the input terminal) to advance one position, and connect the input terminal to the next output.

Many stepping switches had several rows of contacts, one rotating wiper per row. Each wiper would connect to one contact in its row. Which contact was connected depended upon the position. All wipers were aligned to the same position, never at different angles. (The rotating contacts looked, in general, somewhat like the head support arms in a modern rigid ("hard") disk drive.)

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.

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.

Slightly more complicated was the two axis stepping switch (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.

The Stromberg-Carlson X-Y Switch was used for telephone exchanges as well; it was apparently more commonly used 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.

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.)

Stepping switches were invented by Almon Strowger in 1888.

External Link

• Explanation of the way a stepping switch works