keybounce
Multiple recorded keystrokes from a single press of a key on a computer keyboard due to poor electrical contact.
keybounce: electrical noise from a mechanical switch
Keybounce occurs when a single key press registers as multiple separate inputs on a computer system. This happens because the mechanical contacts inside a keyboard switch do not close cleanly in a single motion. Instead, the spring-loaded contact physically bounces against the conducting surface several times over a period of 5 to 50 milliseconds before settling. Each bounce is read as a distinct keystroke by the scanning circuit, resulting in ghost characters or repeated entries where the operator pressed the key only once.
The root cause is mechanical: when a key is pressed, the metal contact leaves its resting position and strikes the receiving contact. If the surfaces are not perfectly flat or aligned, or if contact pressure is insufficient, the contact will separate and reconnect multiple times as residual kinetic energy causes vibration. Poor cleaning or oxidation of the contact surfaces makes the problem worse. Keyboards with rubber domes or mechanical switches can both suffer from keybounce, though quality varies widely between manufacturers.
Detection and mitigation
Hardware and software both address keybounce. The simplest hardware solution is a debounce capacitor placed across the switch contacts, which smooths the electrical signal by filtering out the rapid oscillations. Many industrial keyboards and control panels use debounce circuits built into the controller. Software debouncing, common in modern systems, ignores additional keystrokes registered within a short window (typically 20 to 40 milliseconds) after a key is first detected.
In industrial control systems and data entry workstations, keybounce becomes more than a nuisance. Unwanted duplicate characters can corrupt process parameters, operator logs, or safety-critical data. This is why ruggedized industrial keyboards often feature mechanical switches rated for high cycle count, superior contact plating, and integral debounce circuits. The term itself originates from the literal bouncing motion of the mechanical switch; it has been recognized as a problem since early mainframe era computing.