Creating NAND gates

In this step, we’re going to have a look at how transistors can be combined together to form what are called logic gates, and in particular we’ll look at the NAND gate. We’ve already seen how transistors can be used to represent the value 1 or 0 by acting like simple switches. We’re now going to look at combining these transistors together so they can perform more complex operations.The diagram below shows a circuit containing two transistors. Again you don’t have to understand what is going on in this circuit, just as long as you can understand the basic inputs and outputs.The two switches are acting as inputs to the transistor setup, and the LED represents the output. When both switches are in the off position, the LED is on. Either of the switches can be turned on, leaving the other one off, and the LED still stays on. It’s only when both switches are turned on that the LED will switch off.This arrangement of transistors is called a NAND gate, and you’ll learn about why later.Now, rather than drawing two transistors every time we want to represent a NAND gate, we can use a symbol that looks like this.Whether we use this symbol or show the two transistors, the behaviour of the NAND gate is the same. When both switches are off, the output is on. Turning either of the switches individually doesn’t effect the output. Only when both switches are in the “on” position does the output become “off”.We can represent this behaviour by drawing what is called a truth table, where the state of the switches is represented by 1s and 0s. When both switches are off, the inputs are both 0. Since the lamp is on, the output in this state is 1. If we turn one of the switches to the “on” position, the corresponding input is 1; the lamp remains on, so the output is 1. Only when both inputs are one does the output change to 0.So the full truth table for a NAND gate is as shown below.You can play with your own simulation of a NAND gate on the website CircuitVerse.Here you can select different gates, and choose inputs which can be 1 or 0, with outputs in the form of a digital LED. This will allow you to play around with a NAND gate and investigate the truth table for yourself.You could even have a go at experimenting with different types of gate, which we will be covering in the next step.