Solution:

The Latch as a Contact-Bounce Eliminator:

• A good example of an $\overline{\mathrm{S}}-\overline{\mathrm{R}}$ latch application is in the elimination of a mechanical switch contact “bounce.”

• When the pole of a switch strikes the contact upon switch closure, it physically vibrates or bounces several times before finally making solid contact.

  

• Although these bounces are very short in duration, they produce voltage spikes that are often not acceptable in a digital system. This situation is illustrated in Figure (a).

• An $\overline{\mathrm{S}}-\overline{\mathrm{R}}$ latch can be used to eliminate the effects of switch bounce as shown in Figure (b).

• The switch is normally in position 1, keeping the $\overline{\mathrm{R}}$ input LOW and the latch RESET.

• When the switch is thrown to position 2, $\overline{\mathrm{R}}$ goes HIGH because of the pull-up resistor to VCC, and $\overline{\mathrm{S}}$ goes LOW on the first contact.

  

• Although $\overline{\mathrm{S}}$ remains LOW for only a very short time before the switch bounces, this is sufficient to set the latch. Any further voltage spikes on the $\overline{\mathrm{S}}$ input due to switch bounce do not affect the latch, and it remains SET.

• Notice that the Q output of the latch provides a clean transition from LOW to HIGH, thus eliminating the voltage spikes caused by contact bounce.

• Similarly, a clean transition from HIGH to LOW is made when the switch is thrown back to position 1.