Sangamo KA-21 & WA-21 Synchronous Time Switches  
Written by AnubisTTP on 2006-09-24  


Before the global microprocessor invasion, Sangamo synchronous time switches were the definitive method for controlling exterior lamps and other electrical devices that would need to be powered on and off in accordance with a pre-determined schedule. Sangamo time switches contain a large electromechanical clock movement that drives a rotating metal disk. The disk has a number of movable 'fingers', which varies from model to model: the KA-21 is equipped with six, while the WA-21 has only four. The schedule is set by moving the fingers around the disk to the appropriate time. As the disk makes a revolution, the fingers push mechanical levers which open and close a set of electrical contacts.

Both the KA-21 and WA-21 have a rectangular glass window on their face to verify the set schedule, as well as a smaller, circular window for observing the synchronous motor in operation.


The WA-21 has several extra features which make it superior to the KA-21. Most notable is the WA-21's more sophisticated time display mechanism. The unit also has two secondary dials instead of a singular minutes indicator. Unlike the KA-21, which has a minute indicator that is projected from the surface of the clock, the supplemental indicators on the WA-21 are sealed behind separate glass windows, located above the main dial.

The WA-21 also has a carryover clock, an interesting technological addition. When these devices were in wide use, electric power was not a continuous service. The electric companies would shut down their power plants for an hour or more each night for maintenance. Needless to say, this would cause serious problems for any clock that depended on line current to function. The WA-21 provided a solution to this problem in the form of the carryover clock, a separate spring-driven movement that works in conjunction with the device's electromechanical movement. When powered, the main drive motor is tapped to wind the spring of the mechanical movement. When unpowered, a bi-metal actuator releases the mechanical movement's balance wheel, allowing it to take over until power is restored. The carryover clock can provide backup power to the WA-21 for up to 10 hours.

Sangamo KA-21, serial number 430860, and WA-21, serial number 19662.

KA-21 program mechanism.

KA-21 internal structure, showing complex mechanism for carryover clock.

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