Logic probe prototype





Updated 18/02/2020



In this project we will be building a Logic Probe using Logic Gates. The purpose of this is to test the outputs of a pin to identify whether it is 1 or 0 (HIGH or LOW). This DIY project could be easily made and you can use it to test your circuit functionality according to the expected outputs. Let's get started!


Parts List

1 Red LED

1 Green LED

2 470 Ohm Resistors

1 4011 IC NAND Gate Package

5V Power Supply


Circuit Diagram








Circuit Operation

The key operation of this circuit is to determine whether the digital input on the probe is high or low, and to indicate this. We are using 3 NAND Logic Gates, however the inputs will be cross linked as single inputs. This means that if the input to the NAND gate is 1, the output will be 0, and vice versa.


So, if the logic probe has no input and is not being used to determine a logic level, both LED's will illuminate. When the logic probe has an input level 1, the first NAND gate will send an output of 0 to the second NAND gate which will then output 1 leaving the Red LED illuminated. Also, when the logic probe has an input level 1, the third NAND gate will send an output level 0 to the Green LED, turning it off. On the other hand, if the digital logic pin provides the probe an input of 0, the first NAND gate inverts this producing a 1 for the second NAND gate which inverts it again and turns off the Red LED, and the third NAND gate inverts the 0 to a 1 for the Green LED. When the Red LED is illuminated, this means the logic input is HIGH. And when the Green LED is illuminated, this means the logic input is LOW.


Below is a breadboard video demonstration of how this circuit operates when the probe comes in contact with +5v or GND. We hope you have enjoyed the theory behind this circuit and how you can create your very own DIY Logic Probe Circuit. Stay tuned for more cool projects.



By Zaqyas Mahmood, Electronic Engineer



Breadboard Demonstration