Design Project: Logic Probe
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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