Electronics Projects: How to Build a NOT Gate Circuit

By Doug Lowe

A gate is a basic component of digital electronics. Gate circuits are built from transistor switches that are either ON or OFF. There are a total of 16 different kinds of gates.

One of the simplest of all gate circuits — a NOT gate simply takes an input that can be either ON or OFF and converts it to an output that’s the opposite of the input. In other words, if the input is ON, the output is OFF. If the input is OFF, the output is ON.

Here’s how the circuit works:

  • The input is controlled by a single-pole switch. When the switch is closed, the input is ON. When the switch is open, the input is OFF.

  • The input is sent through the R1and LED1 to bias the transistor. Thus, when the input is ON, LED1 lights up, and the transistor is turned on, which enables the collector-emitter path to conduct. When the input is OFF, LED2 is dark, the transistor turns off, and no current flows through the collector.

  • LED2 is connected directly between the +6 V power supply and ground, through a current-limiting resistor, of course, to keep the LED from burning itself out.

  • The anode of LED2 is connected to the transistor’s collector.

  • When the transistor is off, current flows through R2 and LED2 and the LED lights up, indicating an ON output. But when the transistor turns on, a short circuit is created through the transistor’s collector and emitter. This causes the current to bypass LED2, so the LED goes dark to indicate an OFF output condition.


Project 6-2 shows you how to build the one-transistor NOT gate circuit on a solderless breadboard.


When you complete this project, you should celebrate with great joy that you’ve built your first digital logic circuit.

If you feel like experimenting a bit, here are some suggestions:

  • Try replacing R2 with larger resistors, such as 10 kΩ and 22 kΩ, or insert a 100 kΩ potentiometer in series with R2. What effect does the increased resistance have on the brightness of the two LEDs?

  • Measure the base current and the collector current.

    • To measure the base current, remove the R2 lead in hole F1 and touch your multimeter probes to the lead you removed and the 2X terminal on the switch.

    • To measure the collector current, remove the R1 lead at J15 and touch the multimeter probes to the lead you removed and the LED1 anode.