NAND gate S-R enabled latch
PARTS AND MATERIALS
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4011 quad NAND gate (Radio Shack catalog #
276-2411)
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Eight-position DIP switch (Radio Shack
catalog # 275-1301)
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Ten-segment bargraph LED (Radio Shack
catalog # 276-081)
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One 6 volt battery
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Three 10 kΩ resistors
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Two 470 Ω resistors
Caution! The 4011 IC is CMOS, and
therefore sensitive to static electricity!
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
4, chapter 3: "Logic Gates"
Lessons In Electric Circuits, Volume
4, chapter 10: "Multivibrators"
LEARNING OBJECTIVES
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Although this circuit uses NAND gates
instead of NOR gates, its behavior is identical to that of
the NOR gate S-R latch (a "high" Set input drives Q "high,"
and a "high" Reset input drives Q-not "high"), except for
the presence of a third input: the Enable. The purpose of
the Enable input is to enable or disable the Set and Reset
inputs from having effect over the circuit's output status.
When the Enable input is "high," the circuit acts just like
the NOR gate S-R latch. When the Enable input is "low," the
Set and Reset inputs are disabled and have no effect
whatsoever on the outputs, leaving the circuit in its
latched state.
This kind of latch circuit (also called a
gated S-R latch), may be constructed from two NOR gates
and two AND gates, but the NAND gate design is easier to
build since it makes use of all four gates in a single
integrated circuit.
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