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Understanding logic systems is vital to the work of any Electronic Engineer.
The fundamental thing to understand is that electrical systems have two states – they are either on or off. We call these two states High and Low with High meaning ‘on’ and Low meaning ‘off’. This can also be thought of as a binary (2 state) condition with ‘1’ meaning ‘on’ and ‘0’ meaning ‘off’.
When we are looking at logic systems we commonly use a set of electronic systems called ‘Logic Gates’.
There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR.
Most logic gates have two inputs and one output.
We are going to look at some simple logic gates here but you are free to explore others should you wish to. It should also be noted that Logic gates can be connected together to perform other functions – you could also explore how to connect logic gates together.
An AND gate is one of the more common logic gates.
AND gates have two inputs and one output – we can simulate an AND gate using two switches as the state basically means that if switch 1 AND switch 2 are on then the output is on. Any other state means that the output is off. See Fig 1 below and see if you can build and test this on a prototype board as in Fig 2.
You can also get XOR Gates which inverts the output. You could build an XOR gate using an IC such as the CD4070BE.
The fundamental thing to understand is that electrical systems have two states – they are either on or off. We call these two states High and Low with High meaning ‘on’ and Low meaning ‘off’. This can also be thought of as a binary (2 state) condition with ‘1’ meaning ‘on’ and ‘0’ meaning ‘off’.
When we are looking at logic systems we commonly use a set of electronic systems called ‘Logic Gates’.
There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR.
Most logic gates have two inputs and one output.
We are going to look at some simple logic gates here but you are free to explore others should you wish to. It should also be noted that Logic gates can be connected together to perform other functions – you could also explore how to connect logic gates together.
An AND gate is one of the more common logic gates.
AND gates have two inputs and one output – we can simulate an AND gate using two switches as the state basically means that if switch 1 AND switch 2 are on then the output is on. Any other state means that the output is off. See Fig 1 below and see if you can build and test this on a prototype board as in Fig 2.
You can also get XOR Gates which inverts the output. You could build an XOR gate using an IC such as the CD4070BE.
We can also represent a logic gate like this using something called a truth table (shows when the output is on (1) or off(0)). The truth table for an two input AND gate is shown below in Fig 3.
An OR gate is another one of the more common logic gates.
OR gates also have two inputs and one output – we can simulate an OR gate also using two switches as the state basically means that if Switch 1 OR Switch 2 are on then the output is on. If both of the switches are pressed the output is also on – this is called an Inclusive OR gate. See Fig 4 below and see if you can build and test this on a prototype board (Fig 6). The truth table and symbol are shown in Fig 5.
OR gates also have two inputs and one output – we can simulate an OR gate also using two switches as the state basically means that if Switch 1 OR Switch 2 are on then the output is on. If both of the switches are pressed the output is also on – this is called an Inclusive OR gate. See Fig 4 below and see if you can build and test this on a prototype board (Fig 6). The truth table and symbol are shown in Fig 5.
So far it has been relatively easy to simulate logic gates using switches. When we start to look at more complicated logic it is more effective to use Integrated Circuits (IC’s) which have logic gates built into them.
A NOT gate for example which is also known as an inverter can create an opposite state in a device.
The following circuit (FIG 7) uses an SN74HC04 (Data Sheet) IC to create a NOT gate - when it has no signal the LED is on but by pressing the switch (giving it a signal) the LED turns off, in essence opposite or inverted.
Try building this and research other types of IC’s which you can use for Logic systems.
A NOT gate for example which is also known as an inverter can create an opposite state in a device.
The following circuit (FIG 7) uses an SN74HC04 (Data Sheet) IC to create a NOT gate - when it has no signal the LED is on but by pressing the switch (giving it a signal) the LED turns off, in essence opposite or inverted.
Try building this and research other types of IC’s which you can use for Logic systems.
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