Every picture tells a story
Electronics guide > Digital integrated circuits I > Every picture tells a story

Interestingly, digital circuits of all types (and well cover a few over the next few pages) are made up using just this simple and basic configuration. Even whole computers use this basic inverter as the heart of all their complex circuits.

To make it easy and convenient to design complex digital circuits using such transistor switches, we use symbols instead of having to draw the complete transistor circuit. The symbol for an inverter is shown in Figure 10.4, and is fairly self-explanatory. The triangle represents the fact that the circuit acts as what is called a buffer that is, connecting the input to the output of a preceding stage does not affect or dampen the preceding stage in any way. The dot on the output tells us that an inverting action has taken place, and so the output state is the opposite of the input state in terms of logic level.

The symbol of an inverter  the simplest form of digital logic gate

Figure 10.4 The symbol of an inverter the simplest form of digital logic gate

To aid our understanding of digital logic circuits, input and output states are often drawn up in a table known as a truth table, which maps the circuits output states as its input states change. The truth table for an inverter is shown in Figure 10.5. As youll see, when its input state is 1 its output state is 0. When its input is 0, its output is 1 exactly as weve already hypothesised.

Truth table of an inverter  the circuit of Figure 10.1, and the symbol of Figure 10.4

Figure 10.5 Truth table of an inverter the circuit of Figure 10.1, and the symbol of Figure 10.4

Its important to remember that, in both a logic symbol and in a truth table, we no longer need to know or care about voltages. All that concerns us is logic states 0 or 1.

We can build up a simple circuit on breadboard to investigate the inverter and prove what weve just seen in theory. Figure 10.6 shows the circuit, while Figure 10.7 shows the circuits breadboard layout. The digital integrated circuit used in Figure 10.6 and Figure 10.7 is actually a collection of six inverters, all accessible via the integrated circuits pin connections such an integrated circuit is commonly called a hex inverter, to signify this. Weve simply used one of the integrated inverters in our circuit.

Circuit of an experiment to discover how an inverter works

Figure 10.6 Circuit of an experiment to discover how an inverter works

Possible breadboard layout of the circuit in Figure 10.6

Figure 10.7 Possible breadboard layout of the circuit in Figure 10.6

The integrated circuit used is a type of integrated circuit called a CMOS device, and its in whats known as the 4000 series of integrated circuits its actually known as a 4049. Dont worry about this for now, as well look at more devices in the series (and another series, for that matter) later in the chapter. All you need to know for now is that its a standard dual-in-line (DIL) device, so you must be careful that you position it correctly in the breadboard.

Making sure that the actual integrated inverter used in our circuit has the correct input (that is, either logic 1 or logic 0) is easy. Logic 1 is effectively the same as a connection to the positive battery supply in practice though, you should never connect an input directly to the positive battery supply; instead always connect it using a resistor. On the other hand, logic 0 is just as effectively the same as a connection to the negative battery supply (this time though, its perfectly allowable to connect an input directly). In other words, when the inverters input is connected to positive its at logic 1: when connected to negative its at logic 0.

Its not a good idea to leave an input unconnected (what is called floating), as it may not be at the logic level you expect it to be, so weve added a fairly high-valued resistor to the input of the inverter to connect it directly to the positive battery supply. This means that, under normal circumstances that is, with no other input connected we know that the inverters input is held constantly at logic 1. To connect the inverter input to logic 0, its a simple matter of connecting a short link between the input and the negative battery supply. This isnt shown in the breadboard layout, but all you need to do is add it when you are ready to perform the experiment.

Measuring the output of the inverter is just as easy, and we take advantage of the fact that the output can be used to light up a light-emitting diode (LED). So, when the inverters output is at logic 1, the LED is lit: when the inverters output is at logic 0, the LED is unlit.

Once the circuits built, its a simple matter of carrying out the experiment to note the output as its input changes, and completing its truth table. Figure 10.8 is a blank truth table for you to fill in. Just note down the circuits output, whatever its input state is. Of course, your results should tally exactly with the truth table in Figure 10.5 if it doesnt, youve gone wrong, because as Ive said before in this book I cant be wrong, can I?

Empty truth table for your results of the experiment in Figure 10.6

Figure 10.8 Empty truth table for your results of the experiment in Figure 10.6

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