Thankfully, we have Jack Ganssle, an expert in embedded systems, to help answer this question. So, how long do buttons bounce for? The answer: it varies depending on the switch type. Switches can bounce during both closing and opening operations. Depending on the switch type, the bouncing action may be more extreme for either action.įigure. Yes! Take a look at the bouncing graphs below from an oscilloscope reading of both a switch activation (open to close) and deactivation (close to open). Image from Do switches bounce both when closing and opening? If you counted each of these fluctuations as a button press, you would have very erroneous input!įigure. Indeed, if you look at a switch signal with an oscilloscope, you can see the “bouncing” action-the voltage rapidly fluctuates between LOW (0V) and HIGH (5V) before finally settling into its steady state of HIGH. This “bouncing” action only takes a few milliseconds (rarely more) but can cause problems if you naively assume that every contact is a new button press. Instead, it rapidly “bounces” back and forth before eventually settling into a steady state (see animation above). When a switch/button first hits its contact point, it does not immediately stick. There is a physical mechanism moving to close a connection. When you press a button or slide a switch, you are interacting with an electro-mechanical device. Video from Episode 37 “Contact and Bounce” of The Current Source YouTube channel. Microcontrollers-even old, slow ones like the ATmega328-work so fast that they will read these rapid oscillations as HIGH and LOW input state changes. In comparison to computation, mechanical motion is slow. When a button is pressed or a contact switch moved, it creates a rapid oscillation of open- and closed-circuits before settling to its final state. A slow-motion video of a switch mechanically bouncing off its contacts. Just like the hammer, this electrical contact visibly bounces when first activated, creating a noisy contact signal. The Current Source recorded slow motion videos of switches bouncing during activations and deactivations. We’re going to use the term ‘switch’ to refer to any class of electromechanical device that has electrical contact points that can mechanically move to open or close an electrical circuit. But first, let’s learn a bit more about the problem before discussing solutions.
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So, what can you do? The solution is to “debounce” your switches, which can be done via software or pure hardware solutions, which we’ll address below. A slow-motion video of a hammer bouncing off a table to help illustrate how switch contacts bounce before entering a steady state. As an exaggerated but helpful analog, think of a hammer striking a surface: the hammer will “bounce” with dampened oscillations before entering a steady resting state with a surface. As electromechanical devices, switch contact points are subject to momentum and elasticity, which act together to create rapid contact oscillations (“bounces”) when a switch is “opened” or “closed”.
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In this lesson, we’re going to learn about contact bouncing (or chatter), a common problem with mechanical switches, buttons, and relays, and how to address it. Debouncing solution 3: softening requirements.Debouncing solution 2: using timestamps.Making a debounced button circuit+software solution.
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