A logic probe is a simple and inexpensive instrument contained within a pen-like tube. It has 3 LED indicators that show the status of the point under test.

5 DIY Logic Probes

Continuity Tester with Buzzer – Works on live circuits.

This continuity tester with buzzer works on live circuits. Very important, as common continuity testers are used with the power source disconnected.

Logic probe using 7 segment display

This logic probe with 7 segment display is designed with TTL technology and does not use LEDs to display the high and low logic levels found in many of the logical probes on the market. The circuit shows the capital letter “H” when there is a logical “1” or the letter “L” when there is a logical “0”.

The Wheatstone bridge circuit

This circuit  was designed in 1833 by Samuel Hunter Christie (1784-1865). However, It was Mr. Charles Wheatstone who gave many uses to the Wheatstone bridge circuit when he discovered it in 1843.

Logic Probe using NOT Gates (Inverters)

This logic probe using NOT Gates is implemented with three NOT gates, one rectifier diode, two LEDs and two resistors.

This Two Transistor Logic Probe is a very simple circuit that can be implemented using two NPN bipolar transistors (T1, T2) in a Darlington configuration.