Electronics Engineering (ELEX) Board Practice Exam

If a logic probe indicates a dim display on a gate's output, what could be the underlying issue?

1. The gate is receiving too much voltage

2. The output of the gate appears to be open

3. The gate is functioning normally

4. The probe is defective

The correct answer is: The output of the gate appears to be open

A dim display on a logic probe at a gate's output is indicative of a weak or incomplete signal being produced by the gate. When the output of a logic gate appears to be open, it means that the output is neither firmly at a high logic level (representing a binary 1) nor at a low logic level (binary 0). Instead, it might be floating, leading to an uncertain state that can cause the probe to show a dim illumination, which is often associated with weak signals that are at the borderline of being recognized as a valid logical high. In situations where a gate output is indeed functioning normally, one would expect to see a clearly defined strong high or low signal rather than a dim one. Excessive voltage could potentially damage a gate, but it wouldn’t simply cause a dim display; rather, the output would often be either clearly high or damaged. If the probe were defective, the output would not show any sign at all or fail to provide any reading consistently, rather than showing a dim output. Thus, the indication of the output being open is most likely the reason for a dim display on the logic probe.