contactbounce

Contact bounce, also known as switch bounce, is the rapid making and breaking of electrical contact that occurs when a mechanical switch, relay, or reed switch is actuated. Instead of settling into a single open or closed state immediately, the moving contact flickers against its mate due to mechanical vibrations and surface irregularities. This transient can produce several contact events in a short time before the contact stabilizes.

Causes include the elasticity of the contact springs, inertia of moving parts, surface oxides or contaminants, and the energy delivered by the actuation mechanism. The duration of bounce depends on device design but generally lasts from tens of microseconds to a few milliseconds. In digital circuits, bounce can appear as multiple transitions on an input line if it is not suppressed.

Effects include false triggering or miscounts in digital systems, keyboards, controllers, and measurement equipment. Bounce can also cause arcing or EMI in high-current relays if not managed.

Debouncing techniques aim to prevent these spurious transitions. Hardware approaches include RC low-pass filters, Schmitt-trigger buffers, diode steering for asymmetric charge/discharge, and dedicated debouncer circuits. Material and mechanical improvements, such as better contact surfaces or slower actuation, can reduce bounce.

Software debouncing uses the processor to require a stable reading for a defined interval, typically by periodic sampling and a small state machine or counter. This method adds latency but is inexpensive when a microcontroller is available.

The choice of method depends on response time, power, cost, and the specific switch or relay in use. Contact bounce is a key consideration in designing reliable user inputs, relays, and industrial controls.