Transistor Logic NOT Gate - Inverter

A Transistor-Transistor Logic (TTL) NOT gate or inverter is one of the simplest logic gates used in Digital Electronics. A NOT gate using a transistor is very simple to make. This circuit uses a commonly available bipolar junction transistor (bjt). I tested the circuit implementation using a BC549C NPN transistor, however almost any general-purpose BC range of transistors should work. For example, the BC547, BC548, and BC549, would also work in this circuit.

In a TTL circuit, logic 1 represents +5 V at the input. This causes the transistor to switch ON hard and all the current then passes through its collector to the emitter and into ground. The transistor therefore bypasses the current to the LED, and the LED goes OFF.

When the input is logic 0, represented by 0 V at the input, the transistor stops conducting and goes OFF. All the current therefore goes through the 1 kΩ resistor and through the LED and the LED comes ON.

Inverter Build on Breadboard

Students should always have a go at building logic gate equivalents using transistors because it gives a good insight into how the chip packages operate. This lab-style project shows how a NOT gate is built on a solderless breadboard. It is best to use breadboards because these are simple fun experiments to see for yourself. Breadboards are also very low cost these days and saves having to use expensive solder.

I used a PP3 9 V dry cell as the power source for this experiment. The red wire carries the positive voltage, and the blue wire carries the negative.

The green wire is for the input signal. The input signal is with respect to the negative ground rail in the circuit. For practical purposes, I used a small 1.5 V dry cell for the input signal, with the positive terminal connected to the green wire and negative terminal to the black wire. A small voltage as low as this is enough to make the transistor conduct.

To build this breadboard circuit, you will need a general-purpose NPN transistor such as the BC549. The BC547 and BC548 should also work. Follow their links to see how they are connected.

You will need a commonly available LED with a forward voltage drop between 1.8 V and 2.1 V. You can get these in large bags at places such as eBay. Please refer to the LED Resistor Calculator article to see how to use this component.

I hope that you will already have the rest of the components such as the 1 kΩ resistors.

Transistor-Transistor Logic (TTL) Inverter Operation

As you can see from the operation of this circuit, a small positive voltage at the input switches the transistor ON. When it is fully ON it is said to be saturated. In this condition, all the current through the 1 kΩ load resistor passes through the emitter-collector junctions and into ground. Since all the current passes through the transistor, there is not enough going through the LED, and therefore it goes OFF. Hence, when the transistor is conducting (ON), the LED goes OFF. When the transistor is not conducting (OFF), the LED comes back ON again. This operation is similar to the Boolean NOT function.

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