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Switch Debounce; A mechanical switch bounces or changes state between open and close many times when the switch is moved from one position or the other [contact bounce]. A switch de-bounce circuit inhibits those contact changes from reaching the circuitry reading the switch position. The debounce circuit needs to eliminate all of the mechanical oscillations so that the circuits receiving the signal only sees one circuit open or closer. Other wise the circuit would see dozens of switch changes even though the switch button was only moved once.
Mechanical Relay Bounce
There are three basic ways to debounce a switch; Purchase a stand-alone Switch Controller to handle the problem, Use an RC circuit to damp-out the circuit oscillations, Or if possible read the switch position many times to determine when the switch settles out and stops bouncing. The graphic shows the bounce rate of a mechanical relay, a switch bounce would appear the same.
Microprocessor Control: If the circuit or switch is being read by a uP than it's possible to read the switch after the switch stops bouncing. The code is just written to see a transition and than wait a certain length of time to allow the switch bounce to subside. The uP can also be set up to read the switch position, and then not make a decision until it reads the same state a number of times in a row. For example if the switch position is changed the processor will not make a decision on the current position of the switch until the uP reads the same position hundreds of times. Remember the higher the clock frequency the larger the amount of reads that have to occur, as a switch bounce will last for milliseconds. Switch bounce is a mechanical failure so the bouncing is rather slow.
uP Switch DeBouncer Circuit
Switch Controller: Maxim Integrated Products produces the MAX7347 as a stand-alone Switch Controller. Using another IC produces another line item in the PL, and additional cost. However the switch debouncers may perform other functions to enhance circuit operation. There are also a number of Switch Debouncer IC's that are produced. This is just one example of a switch debouncer.
RC Switch Debouncer Circuit: It's also possible to integrate the switch bouncing [oscillations] so that they are smoothed out [damped out], and the receiving circuit does not see them, but that requires an RC circuit [Resistor/Capacitor circuit]. The Time Constant [TC] of the RC circuit should be greater than the expected time the switch will bounce. An RC circuit which also uses a Schmitt Trigger is shown to the right.
