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Bistable Latch

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AS A2

Bistable Latch / R S Flip Flop

One Bit Memory Circuit

This circuit is commonly shown in three ways.

Left: Two NAND Gates.
Middle: Set and Reset are shown as little circles.
Right: Set and Reset are labelled.

RS stands for ReSet and Set. A zero is needed to set or reset. This is why there is overlining (opposite of underlining) to represent NOT.
Q and Q are the outputs.
If the Set pin is momentarily made zero, one or more times, Q will go high and stay high.
If the Reset pin is momentarily made zero, one or more times, Q will go low and stay low.
The first SET pulse changes the state of the BISTABLE. Subsequent SET pulses have no effect.
The first RESET pulse changes the state of the BISTABLE. Subsequent RESET pulses have no effect.

This circuit can also be used to debounce switches.

The resistors are Pull Ups.
The switches are Pull Downs.
The NAND inputs are high due to the resistors until a switch is pressed. Then the input goes Low.

Bistable Latch

For the Falstad Circuit Simulation, CTRL+Click Bistable Latch
In options, check European Resistors and uncheck Conventional Current.
Click the switches to set and reset. Multiple clicks simulate switch bounce.

Alternatively view Bistable_Latch.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.