Jun 252024

Logic Probe using NOT Gates (Inverters)

This very simple logic probe using NOT gates (inverter gates) consists of three NOT gates, a rectifier diode, two LEDs and two resistors.

The logic probe allows us to know the logic state of any point in a digital circuit. The signal to be measured is applied to the probe tip input.

How does the logic probe work?

When the input signal is “high” (logic 1):

  1. The top NOT gate inverts the signal and the red LED is off.
  2. The diode is directly polarized, the signal is inverted twice by the two cascaded NOT gates, and the green LED is lit.

When the input signal is “low” (logic 0):

  1. The upper NOT gate inverts the input signal and the red LED lights.
  2. The diode is reverse-polarized and does not conduct. As a result, there is a “logical low” at the diode’s anode. This logical low is inverted twice, and the green LED does not light up.

Logic probe using NOT gates

The resistor R2 limits the current flowing through both LEDs (green and red). To implement the circuit, a TTL SN7404 integrated circuit (6 NOT gates in a single IC) is required.


  • A 5-Volt power source powers this circuit and can only be used to test digital circuits manufactured with TTL integrated circuits.
  • The ground (0 volt) connection of the circuit under test and the 0 volt connection of the digital probe must be the same.

List of components for the logic probe circuit

  • 1 TTL SN7404 integrated circuit (6 inverters or NOT gates) (U1, U2, U3)
  • 1 1N4001 or similar rectifier diode (D1)
  • 2 LEDs (red, green) (L1, L2)
  • 1 220 ohms, 1/4 watt resistor (R2)
  • 1 10K, 1/4 watt resistor (R1)

More logic probe circuits:

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