May 312017

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.

The logical probe allows us to know the logical state of any point in a digital circuit. The signal to be measured is applied to the “IN” input.

How the logic probe works?

If the input signal is “High”:

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 lights up.

If the input signal is “Low”:

1- The top NOT gate reverses the input signal and the red LED turns on.
2- The diode is reverse polarized and does not conduct. As a consequence, at the diode anode there is a “logical low.” This “logical low” is reversed twice and the green LED does not light up.

Logic probe using NOT Gates

The R2 resistor limits the current flowing through LEDs (green and red)

To implement the circuit, a TTL SN7404 integrated circuit (6 NOT gates in a single IC) is needed.


  • The voltage source that power this circuit is 5 volts and can only be used to test digital circuits manufactured with TTL integrated circuits.
  • The ground connection (0 volts) of the circuit under test and the 0V terminal of the digital probe must be the same.

List of circuit components

  • 1 TTL SN7404 integrated circuit (6 inverters or NOT gates) (U1)
  • 1 diode 1N4001 (D1)
  • 2 LEDs (red, green) (D2, D3)
  • 1 220 ohm resistor (R2)
  • 1 10K resistor (R1)


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