## Keys in Parallel Diagram

In the picture and diagram above, one need only press either 'key D' OR 'key E' (or both) to turn the light on.

## OR Gate Circuit

In the circuit above, the crossing wires do not touch. Crossing wires never touch! They are never connected. In the circuit above, when 'key D' OR 'key E' (or both) closes, the light comes on. When 'key A' is pressed, then 'key D' closes. When 'key B' is pressed, then 'key E' closes. Therefore, when 'key A' OR 'key B' is pressed, the light turns on. Another way of describing the operation of this circuit is to say that 'output wire C' gets value 1 only when 'input wire A' has value 1 OR 'input wire B' has value 1.

The the following table also shows that 'output wire C' gets value 1 only when either 'input value A' has value 1 OR 'input wire B' has value 1.

```                         OR gate truth table
A       B       C
0       0       0
0       1       1
1       0       1
1       1       1
```

## OR Gate Circuit with Symbol

The diagram above shows a circuit with the symbol for an 'OR gate' which is shown alone, below.

## Three Keys in Parallel

The light in the circuit above turns on when key D, key E, OR key F is pressed.

## Normally Closed Key Diagram

In the picture and diagram above, the light is on, as we have seen before. One must press the normally closed key D down to turn the light off.

## NOT Gate Circuit

In the circuit above, the triangles are both the top of the same battery. When 'key A' is pressed, 'key D' is pulled down and the light goes off. That is, when 'key A' is pressed, normally closed 'key D' opens. Therefore, when 'key A' is pressed, the light goes off. Another way of describing the operation of the circuit is to say that 'output wire C' gets value 0 when 'input wire A' gets value 1. 'Output wire C' gets value 1 when 'input wire A' gets value 0.

The following table also shows that 'output wire C' gets value 0 only when 'input wire A' gets value 1.

```                        NOT gate truth table
A       C
0       1
1       0
```

## NOT Gate Circuit with Symbol

The diagram above shows a circuit with the symbol for a 'NOT gate' which is shown alone, below.

## NOT Gate Symbol

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