Contact Bounce
This is a mechanical contact opening and closing a few times as it settles into stable contact at the moment of closing. The duration is typically between 0.5–10 ms and is invisible to the human eye, but digital inputs with microsecond resolution read it as several separate triggers.
Because contact surfaces are elastic, they bounce back on first touch. During this the circuit opens and closes rapidly. For a lamp or a motor the result is invisible, but a counter input, an encoder channel or a microcontroller reading a button may count a single press as three or four presses.
- Hardware solution: an RC filter made of a capacitor in the order of 100 nF across the contact and a series resistor. The time constant is chosen several times longer than the bounce duration.
- Software solution: ignoring the input typically for 20–50 ms after the first edge. For industrial pushbuttons this value has sufficient margin.
- Symptom: a crane remote control jumping two steps on a single press, a double increment on a counter, a valve chattering briefly and repeatedly.
On relay contacts, bounce also produces an arc; a tiny arc forms at every bounce and the contact surface wears away. For this reason bounce suppression is not only about correct reading but about contact life. On digital inputs where stable operation is required, a semiconductor output may be preferred over a mechanical contact.
Context in which this term is used: Complete guide to IP protection ratings