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  • Logic Probe with Pulse

  • Created: Feb 27, 2014

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This simple circuit is a logic probe with the ability to indicate the presence high voltage, low voltage, and even the presence of a pulse. This circuit will indicate the presence of pulse for a signal with a duration of 200 milliseconds.

The circuit uses the 74ABT00DB quad 2-input NAND gate as the main logic component. The 74ABT00DB controls which of the light emitting diodes (LED) will light up. The three visible indicators: high voltage (logic 1) for the red LED, low voltage (logic 0) for the green led, and pulse for the yellow led. The yellow or ‘pulse’ led comes on for signals with a duration of approximately 200 mSec. The NE555 timer allows the measurement of duration of the pulse for the yellow LED. The yellow LED is directly coupled to the NE555 timer. The circuit is supplied by a 5 volts DC source.

The circuit is good for transistor-transistor logic and CMOS. Logic tracing and signal indication are direct applications for this circuit. The additional capability of measuring a pulse gives the advantage of having more accurate voltage measurements and lessens the incidence of mis-indication.



A logic probe has to be connected to the voltage it is testing. This probe will not work on CMOS

Posted: Jun 28, 2014

Neil Mula

It worked. It is just you need to specify the 555 timer being used like you need to use LM555 instead of NE555 since they have a slight difference in operation.

Posted: Oct 08, 2014



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