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Design Project: Logic Probe
5 questions By Tony R. Kuphaldt
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Question 4 of 5
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.
Reveal answerI’ll leave this part up to you!
Notes:Not only will a discrete transistor have a wider voltage range it can operate over (compared to a logic gate), but it is a good review of transistor circuit theory and a practical example of implementing a simple logic function without the benefit of integrated circuits.
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Question 5 of 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.
Reveal answerNAND gates U4 and U5 form a monostable multivibrator circuit. A low input sensed by U4 at the output of U3 (indicating an indeterminate state) forces U4 to output a high signal, which is inverted by U5 to energize the “Pulse” LED and also hold U4 in that state even when the output of U3 goes high again. This state cannot last indefinitely, though, because the RC network of R7 and C2 brings the input of U5 to a low state over time, thus “resetting” the pulse indication circuit.
Notes:I recommend a 0.47 μF capacitor for C2 and a 100 kΩ resistor for R7. The added feature of a pulse indicator LED is particularly nice because it makes use of what would otherwise be unused gates in a 4011 CMOS NAND gate IC. The only added componentry is the fourth LED, current limiting resistor R6, capacitor C2, and resistor R7.
