Switch Debouncing

In the circuit below when the switch is in position (a) then

= +5V and

= 0V i.e.

= 1 and

= 0 Similarly, when the switch is in position (b) then

= 0V and

= +5V i.e.

= 0 and

= 1

In reality, when the switch moves from (a) to (b) (or vice versa) then two things happen

for a brief moment both and are 0.
The switch may "bounce", that is make and break the connection a number of times before settling correctly on the contact.

This problem can be solved by connecting

to the S input and

to the R input of an SRFF. In this case, once

has made the transition from 0 to 1 this is equivalent to the Set feature of the SRFF. As has been seen, once this has happened no matter how many times S toggles between 0 and 1 then the output remains at 1 until R is set to 1. Hence once the first contact is made the output remains stable despite the subsequent bounces of the switch and so the circuit behaves as intended. This is illustrated in the timing diagram below.

PHY107 Home