How to build XOR gate with relay ???

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Hi Guys…
I’ve dug some old file from my collage time and i found this
I think it’s cool enough to be shared with you all…
Check this out…

Case:

It was given that i need to install a circuit consists of 2 SPDT switches and 1 light bulb.  Let’s say switch A and switch B. The sequence is as below

State 1, No switch is altered, the bulb is off

State 2, Either switch A or B is altered, the bulb is on

State 3, Either switch A or B is altered again, the bulb is off

Solve it using relay circuit !!!

Solution:

That case could be done by XOR logic gate. Check out this XOR table below.

A   B   Out

0   0     0     Previous state

0   1      1     Only switch B status is altered

1   0      1     Only switch A status is altered

1    1     0     Both switches status are altered

Check out the circuit below (Click to enlarge)

You can build the circuit using 2 SPDT relays.

When only switch A (or B)  is altered, relay A is active thus the NO become close circuit and the NC become open circuit. Then the current can flow as has been shown with the red arrow.

When no switch is altered (or both switches are altered together), the light bulb will go off because of the NO/NC nature of the relays (Notice the red circle, how the NC become open circuit when the relay is activated).

let’s compare the result of above solution with the sequence asked by the case.

State 1, No switch is altered, the bulb is off <— TRUE

State 2-1, Switch A is pressed, the light bulb is on

State 3-1, Switch B is pressed, the light bulb is off (Both switches are pressed)

State 3-2, Switch A is pressed again, the light bulb is off (No switch is pressed)

or

State 2-2, Switch B is pressed, the light bulb is on

State 3-1, Switch A is pressed, the light bulb is off (Both switches are pressed)

State 3-2, Switch B is pressed again, the light bulb is off (No switch is pressed)

State 2-1 and 2-2 meet the requirement of  state 2 in the case above, so it’s TRUE.

State 3-1 and 3-2 meet the requirement of  state 3 in the case above, so it’s also TRUE.

Then the case is solved

Cheers,
Edwin

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How to build NOT gate with relay