## Clear Key Diagram

The picture and diagram above show a loop as before, but a normally closed key has been added. As long as the normally closed key is closed, the loop works as before.

However, if the normally closed key is pressed, then the normally closed key will be open and electricity will not reach the electromagnet, so the electromagnet will not be magnetic, and the normally open key will pop up if it was down. If the normally open key already was up, it will stay up.

Therefore, pressing the normally closed key will clear the value of the loop to '0.' Therefore, this normally closed key is called the 'clear key' for the loop.

## Loop to Loop Data Transfer

In the circuit above, the 'connecting key' connects loop A and loop B. Both loops have value 0. Temporarily pressing 'loop key A' gives the value 1 to loop A. Now, temporarily pressing the 'connecting key' will make loop B have value 1. That is because when loop A has value 1, loop key A is closed, loop wire A has value 1, and when the connecting key is closed, electricity can reach the electromagnet of loop B, giving loop B value 1.

However, if loop A has value 0, and loop B has value 0, and the connecting key is pressed, then both loops keep their values of 0.

Therefore, if one temporarily presses 'clear key B' to clear loop B to value 0, and then temporarily presses the connecting key, whatever value is in loop A will be copied to loop B. Then loop A and loop B will have the same value.

## Oscillator Diagram

The picture and diagram above show a normally closed relay that controls its own electromagnet. The square of wire that takes electricity from the normally closed key of the relay to the electromagnet of the same normally closed relay is called a feedback wire. (Notice that this circuit is different from a loop circuit, which uses a normally open relay.) This circuit is called an oscillator because the relay oscillates (changes back and forth) between open and closed.

Electricity can get from the top of the battery, through the closed, normally closed relay key to the electromagnet. The electromagnet then pulls the normally closed key down and opens the normally closed key. Because the normally closed key is now open, no electricity can get to the electromagnet. The electromagnet now no longer attracts the normally closed key and the normally closed key closes.

Thus, the normally closed key repeatedly opens and closes without anyone touching the key. The feedback wire gets value 1, then value 0, then value 1, etc. It takes a relay about a hundredth of a second to change values.

Just as a normal loop is the basis of a computer memory, this feedback circuit is a key part of a computer's clock. A computer's clock is a circuit that repeatedly generates signals (1 and 0 values).

## Keys in Series Diagram

In the picture and diagram above, one must press both 'key D' AND 'key E' to turn the light on.

## AND Gate Circuit

In the circuit above, the three triangles are all the top of the same battery. When 'key D' AND 'key E' close, then 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' and 'key B' are pressed, the light turns on. Another way of describing the operation of the circuit is to say that 'output wire C' gets value 1 only when 'input wire A' gets value 1 AND 'input wire B' gets value 1.

The bottom row of the following table also shows that 'output wire C' has value 1 (only) when both 'input wire A' has value 1 AND 'input wire B' has value 1.

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

## AND Gate Circuit with Symbol

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

## AND Gate Symbol

The light in the circuit below only comes on whey key D, key E, AND key F are all pressed.

## Three Keys in Series

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