Stepping switch

In electrical controls, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical pulses.

The major use of stepping switches was in early automatic telephone exchanges to route telephone calls. Later, they were often used in such equipment as industrial control systems. They were used in Japanese cypher machines during World War 2, known to the Americans as CORAL, JADE, and PURPLE. Code breakers at Bletchley Park employed uniselectors driven by a continuously rotating motor rather than a series of pulses in the Bombe machines to cryptanalyse the German Enigma ciphers.

In a uniselector, the stepping switch steps only on one axis, although there are often several sets of contacts in parallel. In other types, such as the Strowger switch, mechanical switching occurs on two directions, across a grid of contacts. The Strowger switch was invented by Almon Brown Strowger in 1888.

Stepping switches were widely used in telephony and industrial control systems (among related applications) when electromechanical technology was paramount.

A basic stepping switch is an electrically operated rotary switch with a single (typically input) terminal, and multiple (typically output) terminals. Like other typical rotary switches, the single terminal connects to one of the multiple terminals by rotating a contact arm, sometimes called a wiper, to the desired position. Moving from one position to the next is called stepping, hence the name of the mechanism. Using traditional terminology, this is a single-pole, multi-position switch.

While some stepping switches have only one pole (layer of contacts), a typical switch has more; in the latter case, all wipers are aligned and move together. Hence, one input with multiple wires could be connected to one of multiple outputs, based on the receipt of a single set of pulses. In this configuration, the rotating contacts resembled the head support arms in a modern hard disk drive. Multipole switches were common; some had perhaps as many as a dozen poles, but those were less common.

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