Skip to content

Button Bounce Oscilloscope

Run the Button Bounce Oscilloscope MicroSim Fullscreen
Edit in the p5.js Editor

About This MicroSim

When a button's metal contacts close, they bounce apart and together several times in a few milliseconds. A program fast enough to notice sees each bounce as a separate press. Debouncing ignores those extra edges, either in software with a time window or in hardware with a capacitor.

This MicroSim works like an oscilloscope set to single-shot: each Press button captures the pin voltage starting 5 ms before the first falling edge. Every press makes a new random bounce of 3 to 13 contact changes over 1 to 15 ms. Triangles under the trace mark each falling edge the Pico sees: green if the software counts it, red if the debounce window rejects it. The shaded blue band is the window after each accepted edge. Move the Debounce window slider until each press counts once, and try the Hardware filter, which smooths the bounce so only one clean edge reaches the pin.

This MicroSim supports the chapter Buttons, Interrupts, and State Machines.

How to Use

  1. Press Press button several times. Compare "Physical presses" with "Presses counted".
  2. Move the Debounce window slider. The markers and the verdict update for the current capture, and new presses are counted with the new window.
  3. Check Hardware filter (0.1 uF capacitor) to see the smoothed trace and the input thresholds.
  4. Use the Span menu to zoom in (20 ms) or out (150 ms). Press Reset count to start counting again.

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

1
2
3
4
<iframe src="https://dmccreary.github.io/clocks-and-watches/sims/button-bounce-scope/main.html"
        height="547px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Learning Objectives

Students will be able to:

  • Analyze how contact bounce turns one press into several falling edges
  • Evaluate how long a software debounce window must be to count each press once
  • Compare software debouncing with a hardware RC filter

Grade Level

9-12 (high school)

Duration

10-15 minutes

Prerequisites

  • Pull-up resistors and active-low buttons from Chapter 5
  • ticks_ms() and ticks_diff() from Chapter 6

Activities

  1. Exploration (4 min): With the window at 0 ms, press the button ten times. Record how many presses are counted.
  2. Guided Practice (6 min): Find the smallest window that counts every press once over ten presses. Compare it with the longest bounce you saw.
  3. Evaluate (5 min): Argue for a window between 20 and 50 ms. Explain what goes wrong with 0 ms and with a very long window such as 300 ms.

Assessment

  • Students choose a window longer than the longest bounce and justify it with data from the scope.
  • Students explain why the hardware filter produces one falling edge.
  • Students relate the slider to DEBOUNCE_MS in the chapter's code.

References

  1. Switch: contact bounce - Wikipedia section on why contacts bounce
  2. A Guide to Debouncing (Jack Ganssle) - measured bounce times from real switches and debounce methods
  3. RC time constant - Wikipedia article on tau = R x C used by the hardware filter