Switches are intended for just turning ON or OFF the power supply by either closing or opening the circuit.
Not really. Switches in process control are intended to sense a state. That the result and its use can be combined is rather a nice shortcut.
Already early on, contactors were used to control power machienery with lower voltage circuits, itself controlled by series of switches, either direct handeled, or by prior contactors - like having Motor B only running if A is already running and up. Same is true for mechanical constructions.
I often wonder how does mankind got idea to use these switch for sake of processing (simple processing like motor control) even before the thought of using Boolean algebra for analysing switch networks in mid 1930s.
Relais are around since the 1820s and have been used for logic combinations since short thereafter - way before Boole wrote his book in the 1840s. Practical use of logic has been around since maybe the begin of time - and documented since Roman times.
Each Cuckoo clock is a piece of staggered logic for process control. More so every tower clock ringing bells accordingly at the hour for the hour.
Bools book is an exercise to formalize logic notation and fix its notation as well as rules. History goes way before - as well as it's important to keep in mind, that in the way we understand it today, it was developed in the 1900s to 1930s. And it wasn't until after Shannon that it became a valid tool for engineering.
What are some early applications which used switches not just mere purpose of turning ON or OFF the circuit but for processing?
Turning on and off is an inseparable result of processing. Even no matter how trivial it may seam. And all of this processing has been used way before any electricity.
Think of a lever moved by water level, pressing a plug into a tube when the level gets to low, thus saving a mill from getting overflown and damaged when stuck because of not generating enough power to drive whatever machinery. Clearly some (analogue) processing of water level vs. power requirement.
How may be these early devices be developed without aid of Boolean algebra?
Take above lever and plug and replace it by a switch and solenoid. Or as control circuit for the generator, switching of users when power level slides, or many alike.
- Logic is independent of implementation,
- Implementation can be done in many different ways, analogue and digital, electrical and mechanic, and finally
- Boolean Algebra is only a way to express logic in as a formal system, not precondition nor result of processing.
Bottom line: It's not applicable to condition on independant areas.