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Simple Logic Level Indicator Circuit

Simple Logic Level Indicator Circuit

This logic detector probe circuit can be very useful device for those who may want to measure the logic levels of digital circuits frequently. Being an IC based circuit, it's implemented in CMOS technology, its aplication is more dedicated to test circuits using the same technology.

By: R.K. Singh

Circuit Operation

The power for the proposed logic tester is obtained from the circuit under test itself. However care must be taken not to put the power terminals in reverse, so when it is connected make sure to set the colors of each of the connecting wires.For example: Red Color, for the cable that connects with the positive voltage (CN2) and black color to the wire that goes to 0 volts. (CN3)

Operational details of logic tester probe with IC 4001

The operation is very simple. The 4001 CMOS integrated circuit has four two-input NOR gates, 3 LEDs and a few passive components used in the design.

Implementation also becomes crucial so that it is comfortable to apply while testing, therefore the printed circuit should be in the elongated in shape preferably.

Looking at the figure we see that the sensing signal is applied to CN1 terminal, which is connected to a NOR gate, whose inputs are in turn connected as a NOT gate or an inverter.

The inverted signal is applied to the 2 LEDs. The diode is switched depending on the voltage level (logic) at the output of the gate.

If the input is high logic level output of the first gate goes low activating the red LED.

Conversely if the detected is low, the signal is sensed is as a low level, the output of this gate is then rendered at high level illuminating the green LED.

In the event if the input signal is an AC or pulsing (varying voltage level constantly between high and low), both red and green LED light become on.

To acknowledge that a pulsed signal may be sensed, the yellow LED starts flashing here. This flashing is executed with the use of the second and third NOR gates, C1 and R4 which functions like an oscillator.

The oscillator output logic is applied to a 4th NOR gate connected as inverter gate which is directly responsible for activating the yellow LED via the given resistor. This oscillator can be seen continuously triggered by the output of the first NOR gate.

Circuit Diagram

Parts List for the above explained logic tester probe circuit

- 1 Integrated circuit CD4001 (4 2-input NOR gate CMOS version)
- 3 LEDs (1 red, 1 green, 1 yellow
- 5 resistors: 3 1K (R1, R2, R3), 1 2.2M (R5), 1 4.7M (R4)
- 1no capacitor: 100 nF


About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

12 thoughts on “Simple Logic Level Indicator Circuit”

  1. Any gate other than the 4000 10v series doesn’t work, I can confirm that. The circuit also doesn’t seem to work on if the supplied voltage is less than 10 V. I tried everything from changing resistor values to altering the circuit itself and no luck. Correct me if I’m wrong.

    Tests where done using multisim btw.

    • Only a practical test can fully confirm the results, I have played with many logic circuits in my experiments so far and I have seen they usually respond well to even less than 5V inputs, so this one should also work with 5V easily,

      You can detach the gates on the other side of the two LEDs and check the response without those 3 gates, if still the LeDs do not work below 10V, then possibly the simulator is producing the wrong results.

            • I mean to say, where is the input probe which can be used for feeding the logic levels to the circuit?

              In the original circuit, CN1 is the input probe which can be touched to any external logic source and for the getting the results on the LEDs

  2. I’m going to attempt rebuilding this circuit for a project however I’ve got some questions:
    1) How much voltage should I apply for this circuit to work?
    2) Why did you use NOR gates instead of just using NOT getes?

    • 1) any voltage between 5V and 15V will work
      2) 4001 IC will have better features than any NOT gate IC, although NOT gates can be also used

      • Couple more questions if you don’t mind:
        1) Will I be able to measure TTL based logic gates using this circuit?
        2) If I tried rebuilding this circuit but instead using 7402 logic IC, what alternations should be done?

        • 1) yes TTL logics can be also checked
          2) No changes would be required, just make sure to keep the supply voltage precisely at 5V

          • I know I must be getting annoying by now, but can you please do a brief explanation about how each NOR gate contributes to the function of this circuit? It’ll be great help for me and future visitors of this article.

            Thanks in advance,

            • Hi Basel, this was not designed by me, still I can somewhat simulate it.
              The input gate is configured as a fast unstable oscillator, the center two gates are configured as timers, while the extreme right side gate is positioned as a buffer.

              However I am not quite sure regarding the function and purpose of these specific stages

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