Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download


Switch De-Bouncing

The fact that repeated pulses at the Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE) inputs are ignored after the initial pulse has set or reset the Q output, makes the SR Flip-flop useful for switch de-bouncing.

When any moving object collides with a stationary object it tends to bounce; the contacts in switches are no exception to this rule. Although the contacts may be tiny and the movement small, as the contacts close they will tend to bounce rather than close and stay closed.

 

          Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE)

This causes a number of very fast on and off states for a short time, until the contacts stop bouncing in the closed position. The length of time of the bouncing may be very short, as shown in Fig. 5.2.3 where a number of fast pulses occur for about 2ms after the switch is initially closed (red arrow). For many applications this switch bounce may be ignored, but in digital circuits the repeated ones and zeros occurring after a switch is closed, will be recognised as additional switching actions.

 Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE)

 

Switch De-Bounce Circuit 

The SR flip-flop is very effective in removing the effects of switch bounce and Fig 5.2.4 illustrates how a SR flip-flop can be used to produce clean pulses using SWI, which is a ‘break before make’ changeover switch. When SW1 connects the upper contact to 0V, the S input changes from logic 1 to logic 0 and R is ‘pulled up’ to logic 1 by R1.

As soon as Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE) is at logic 0, (at time ‘a’ in Fig. 5.2.4) output Q will be at logic 1 and any further pulses due to switch bounce will be ignored.

When SW1 is switched to the lower contact, there will be a short time (between times ‘b’ and ‘c’ in Fig. 5.2.4) when neither Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE) or Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE) is connected to 0V. During this time Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE) returns to logic 1, therefore both inputs will be at logic 1 until time ‘c’, when SW1 connects R to 0V and Q is reset to logic 0 completing the output pulse. The use of a ‘break before make’ rather than a ‘make before break’ switch is important, as it ensures that during the changeover period (time ‘b’ to time ‘c’ in Fig. 5.2.4) both inputs are at logic 1 rather than the non-allowed state where both inputs would be logic 0. This ensures that outputs Q and Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE) are never at the same logic state.

Although, during the change over of SW1 both inputs are at logic 1, this does not produce the indeterminate state described in Table 5.2.1, as one or other of the inputs is always at logic 0 before both inputs become logic 1.

  Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE)

The document Switch De-Bouncing: S-R Flip Flops | Analog and Digital Electronics - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Analog and Digital Electronics.
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FAQs on Switch De-Bouncing: S-R Flip Flops - Analog and Digital Electronics - Electrical Engineering (EE)

1. What is switch de-bouncing?
Switch de-bouncing refers to the process of eliminating or reducing the undesired effects caused by the mechanical bouncing of a switch when it is pressed or released. When a switch is operated, it can produce multiple rapid open-close transitions, which can lead to false triggering or erratic behavior in digital circuits. De-bouncing techniques are used to ensure that only a single stable output is obtained for each switch operation.
2. How does an S-R flip flop help in switch de-bouncing?
An S-R flip flop is a type of sequential logic circuit that can be used for switch de-bouncing. It has two inputs: the Set (S) and Reset (R) inputs. By connecting the switch to the S input and its complement to the R input, the flip flop can be used to latch the desired output state when the switch is stable, effectively eliminating the effects of switch bouncing. This stable output can then be used as an input to other circuits.
3. What are some common de-bouncing techniques other than S-R flip flops?
In addition to S-R flip flops, there are several other commonly used de-bouncing techniques. One popular approach is to use a simple RC circuit, where a resistor (R) and capacitor (C) are connected in parallel with the switch. This RC network acts as a low-pass filter, smoothing out the bouncing transitions. Another technique is to use software de-bouncing, where the bouncing effect is filtered out using algorithms in the microcontroller or digital signal processing software.
4. Why is switch de-bouncing important in digital circuits?
Switch de-bouncing is important in digital circuits because it ensures reliable and accurate operation of the circuit. Without proper de-bouncing, the bouncing effect of the switch can lead to false triggering, multiple inputs, or unpredictable behavior in the circuit. This can result in errors, data corruption, or even damage to the circuit components. De-bouncing techniques help to ensure that the circuit responds correctly to switch operations, providing stable and reliable outputs.
5. Can switch de-bouncing be necessary for all types of switches?
Switch de-bouncing is not necessary for all types of switches. It is typically required for mechanical switches that have physical contacts, such as push buttons or toggle switches. These types of switches are prone to mechanical bouncing due to the nature of their operation. On the other hand, electronic switches, such as solid-state relays or optocouplers, do not suffer from mechanical bouncing and therefore do not require de-bouncing.
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