This circuit has memory. It remembers whether it is in the set or reset state. It can be used as a latch. Once set, it remains set until a reset signal clears the memory.

Alternative Diagrams

Note the inverting inputs on the left hand diagram. (Not Knobs).

• 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 NOT 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.

De-bouncing

When switches operate, the contacts don't just connect and disconnect. They bounce several times before settling in the on or off position. This doesn't matter if you are turning on a light. If you are counting pulses using a circuit that can detect 4 000 000 pulses per second, switch bounce becomes important.

The RS Flip Flop can be used to de-bounce a switch. Here is the circuit.

• When the switch S is pressed, SET goes low. This sets Q high. The flip flop LATCHES (it remembers). If the switch contacts bounce, Q is set on the first bounce and subsequent bounces have no effect. When the switch is released, Q remains set.
• When the switch R is pressed, RESET goes low. This sets Q high. The flip flop LATCHES (it remembers). If the switch contacts bounce, Q is set on the first bounce and subsequent bounces have no effect. When the switch is released, Q remains set.

 

Another De-bouncing Technique

The circuit below also de-bounces the switch. When the switch is pressed, C is discharged. The gate input goes low (to zero). If the switch contacts bounce, the capacitor does not have time to re-charge so during the bounces, the gate input stays low. When the switch is released, the capacitor charges up. When the gate input is about V_s / 2, the gate input changes state and goes high. This technique is simpler than the flip-flop method above but also potentially less reliable. When the gate input crosses V_s / 2, there is a small risk of the gate picking up electrical noise. This can be avoided by using a gate that includes s Schmitt Trigger. (right hand circuit).

Or Gate Latch / Bistable

When neither switch is pressed, b will be low.

1. If q is high, then a will be high.
   If a is high, then q will be high.
   This is a stable state
2. If q is low, then a will be low.
   If a is low, then q will be low.
   This is the other stable state

If the reset switch is pressed, b is low and so is a.
    The output q will go low and stay low even if the reset switch is released.
    See state 2 above.

If the set switch is pressed, b goes high.
    The output q will go high and stay high even is the set switch is released.
    See state 1 above.

Questions ...

What happens if both switches are pressed. Also what happens is Set is released first and what happens if Reset is released first.

Is switch contact bounce a problem with this circuit?