Basic Gates Using NO and NC

In an industry there can be many switches and sensors together forming a control system for a single output as well as there are major possibilities that most of our outputs might be connected to each other i.e. the status of one output might be turning off another.

The sensors and switches have only two states so they provide discrete signals i.e. high (1) or low (0). Their status can be referred as inputs if used to control a certain output. Switches connected in different sequence might result in a distinct process control. Now we do know some logic gates that turn the output ON only if certain conditions of the inputs are found true such as AND gate, OR gate, NOT gate. Let us go through them and figure out the Relay logic that can be designed for such logic.

To simplify and make it easy to understand we would be only considering two inputs for now(i.e. only two switches).

AND Gate:-

The output turns high only if all the inputs are high.

A . B = Output

Checking out the truth tale it clearly notifies that the output turns high(1) only when both the inputs turn high.

If we are using two switches for inputs then we can say that the output should turn ON only when both the switches are pressed together.

RLD for AND gate

For example let us consider a ceiling fan and the connections are made such that we need to press two switches to turn it ON that simply tells that the current might pass through one switch when it is pressed but it needs the other switch pressed to reach the fan, i.e. is they might be connected in series

Let us check the following RLD for AND gate and compare it with the Truth-Table.

Checking out the RLD it explains that only when both of the switches are pressed the output coil will turn ON. Even when we compare to the truth table to RLD circuit we can conclude that the input that gets high value is pressed

Concluding that if the value of a input is high that signifies that the switch is pressed in RLD similarly 0 is for released (no action)

 i.e.   1(high) in truth-table = switch pressed

      0(low) in truth-table = switch released (no action)

OR Gate:-

The output turns high if any of the input is high.

A + B = Output

If we are using two switches for inputs then we can say that the output should turn ON when any of the switch is pressed that is one is bypassing the other

For the same celling fan and two switches the connections are different as any one of the switch can turn on the fan i.e. there are two different paths and current flow in any of them can turn on the output. Simply the switches are connected in parallel so that one switch can bypass the other.

The Relay Logic Design clarifies more of it

A new conclusion can be drawn to clarify further Relay Logic designing.

AND = A . B = Output = Series

OR = A + B = Output = Parallel

OR Gate:-

Also called as inverter for its specific function it uses a single input the status of the O/P is always inverted form of the input i.e. if input is high output is low and vice versa.

Inverted A = B

Speaking about a switch whenever the switch is pressed (activated/energized) the output is low and whenever the input is released (de-energized/no action taken) the output in ON.

In electrical the NC switch works in the same way as the word itself tells that it is connected in normal condition i.e. when the switch is neither pressed nor activated it works as an closed contact and when pressed it breaks the contact resulting open circuit and hence turning the output OFF.

Here we can conclude with a very important logic that is necessary to draw out the Relay logic for each and every case even if there are multiple inputs or multiple output.

The logic delivers an interconnection between the Boolean equation, the status of the switch (1 or 0 i.e. pressed or released) and type of switch (NO and NC).

We will see some more example of that while designing relay logic for universal gates in the upcoming topic(universal gates)

In case you have any queries

contact me