It is almost impossible to obtain a “clean” voltage transition from a mechanical switch because of the phenomenon of contact bounce. This is illustrated in Figure 5-9(a), where the action of moving the switch from contact position 1 to 2 produces several output voltage transitions as the switch bounces (makes and breaks contact with contact 2 several times) before coming to rest on contact 2. The multiple transitions on the output signal generally last no longer than a few milliseconds, but they would be unacceptable in many applications.

A NAND latch can be used to prevent the presence of contact bounce from affecting the output. Describe the operation of the “switch debouncing” circuit in Figure 5-9(b).

Figure 5-9

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+5 V +5 V www +5 V C ww-II IS R O -VOUT (a) VOUT (b) 5 V OV 0 Random "bouncing" www Switch to position 2 Switch to position 2 Switch comes to rest in position 2 Switch back to position 1