Design Project: Logic Probe

Digital Circuits

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  • Question 1

    What factors determine where potentiometers Rpot1 and Rpot2 must be set? Why are they “trimmer” potentiometers (as indicated by the special wiper symbol) and not regular panel-mount potentiometers?

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  • Question 2

    What performance parameters are important to consider for U1 and U2, considering their use in a logic probe circuit? Hint: we may want to use this logic probe to troubleshoot CMOS as well as TTL circuits.

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  • Question 3

    What purpose do resistors R1 and R2 serve, and why are they so large (1,000,000 ohms each)?

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  • Question 4

    Re-design the example circuit so that a NAND logic gate is not required. Instead, think of a way you could use discrete components to do the same job.

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  • Question 5

    An extra feature you could add to the logic probe circuit is a pulse indication LED. This LED momentarily turns on whenever there is a transition from high-to-low or from low-to-high:

    Actually, what the pulse indicator circuit detects is a transition to the “indeterminate” state, which always lies between “high” and “low.” A pulse indication feature is nice to have in some circumstances, since it shows the presence of pulses which may be too brief to light up either the “high” or “low” LED. The two additional NAND gates “stretch” the pulse time so that the “pulse” LED’s blink is long enough to see. The duration of the LED’s blink is set by resistor R7 and capacitor C2.

    Explain how the pulse indication circuitry works.

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