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Pull-Up Resistor and Button Simulator

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About This MicroSim

Wire a push button to a Pico pin and you would expect a clean answer: pressed or not pressed. Leave out one small resistor and the pin instead reports pure nonsense. This simulator starts with the resistor switched off so you see that problem first, then lets you switch it on and watch the reading go rock steady. Your goal is to explain why an undriven input pin is called floating, and to demonstrate that a pull-up resistor turns a floating pin into a reliable HIGH.

How to Use

  1. Look at the readout before touching anything. The pull-up is OFF and the value flickers between HIGH and LOW on its own, because nothing is driving the pin.
  2. Hold PRESS AND HOLD BUTTON. The flicker stops and the pin snaps to LOW, because the closed button ties GP15 straight to ground.
  3. Let go. The flicker returns. That is the problem a pull-up resistor solves.
  4. Click Pull-up resistor: OFF to turn it ON. The resistor symbol turns red and the released reading holds steady at HIGH.
  5. Hold the button again with the pull-up ON. The pin still reads LOW, so both states are now trustworthy.
  6. Click Reset to return to the starting conditions and try it again.

Iframe Embed Code

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

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<iframe src="sims/pull-up-resistor-simulator/main.html"
        height="482px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Grade Level

9-12 (High School)

Duration

10-15 minutes

Prerequisites

  • Knows that a GPIO pin can be set as an input that reads a voltage.
  • Understands HIGH and LOW as the two digital values a pin can report.
  • Can read a simple schematic with a supply rail and a ground rail.
  • Knows that the Pico runs its logic at 3.3 volts.

Activities

  1. Exploration (5 min): With the pull-up OFF, watch the readout for ten seconds without clicking. Write down what the value does and why that is a problem for code.
  2. Guided Practice (5 min): Fill in a four-row table for all four combinations of pull-up ON/OFF and button pressed/released. Record the state name for each row.
  3. Assessment (5 min): Predict the reading for "pull-up ON, button released" before you set it up, then test your prediction and explain any surprise.

Assessment

  • Correctly reports the pin state for all four stages of the table.
  • Defines a floating pin as an input with no circuit driving it to a known voltage.
  • Explains that the pressed button always wins because it connects the pin to ground.
  • States one real consequence of a floating pin in robot-face code, such as an expression that changes when nobody pressed anything.

References

  1. Pull-up Resistor (Wikipedia) - Definition, circuit role, and typical resistance values.
  2. MicroPython machine.Pin Documentation - The Pin.PULL_UP argument that enables the Pico's internal pull-up resistor.
  3. Raspberry Pi Pico Documentation - GPIO electrical characteristics for the RP2040 microcontroller.
  4. Push Button Switch (Wikipedia) - How a momentary switch opens and closes a circuit.