Inverter or NOT Logic Gate

An inverter is a type of logic gate and is also called a not gate. It takes an input that is off and turns it on or takes an input that is on and turns it off. This is actually useful in many cases. Some people use an integrated circuit to implement an inverter but it can be built easily using just one NPN transistor.


The photo above shows an inverter that I built on a breadboard. Right now the input is off as the button is not being pressed. This is why the output which is represented by the LED is turned on. When the button is pressed current and a positive voltage is applied to the base of the transistor. This makes the input on and the LED will turn off. The LED turns off because the current is allowed to flow from the collector to the emitter of the transistor. This makes a 4.948 Volts voltage drop across the 2K resistor going into the collector. When this happens there is only .016 volts left to try and turn on the LED. This will not turn on the LED as 1.9 volts is required to turn it on and let current flow across a yellow LED.

The video above shows how to build all types of logic gates including an inverter. At 5:35 in the video, the inverter is discussed in detail. It is good to know how to build all the different types of digital logic gates and I recommend watching the entire video.

Inverter Truth Table

inverter truth table

The symbol and truth table for the inverter is shown above.  Its symbol is similar to a buffer but it has a circle at the front pointing out that it is the opposite logic of a buffer. Looking at the truth table you can see there is just one input. When the input says 0 that means that it is off. When the input is 1 that means that it is on. The inverter takes the input and makes the output the opposite.

Inverter Circuit Diagram


There are multiple ways that an inverter can be implemented. The circuit diagram above shows the previous example where a push button switch controls if the input is on or off. This is just for demonstration purposes and in most cases, a push button switch is not used in an inverter circuit. The input can come from a multitude of sources including, another part of the circuit, or a separate circuit. The input can also be connected directly to the base of the transistor from the positive 5-volt rail using the 2K resistor.

Leave a Comment