Stage 2 · Lesson 5 of 7
Pull-ups, pull-downs and buttons
An input pin connected to nothing floats: it picks up noise and reads random values. A resistor ties it weakly to the supply so it reads high until something pulls it low; a pull-down does the opposite. A button between the pin and ground with a pull-up reads high when released and low when pressed.
Most have internal pull-ups you can enable in code, and lines need pull-ups as well, which you will meet in the protocols stage. Typical external values are a few kilohms to tens of kilohms.
Common trapwiring a button with no pull resistor at all, then chasing an input that changes when you move your hand near it, which feels like a software bug but is physics.
A question to check yourself, an AI practice session and 1 cheat sheet entry come with the full roadmap. Unlock
Resources for this stage
Start with
- BookFreeAll About Circuits Textbook
All About Circuits · Beginner
Free: read the DC volume chapters on Ohm's law, series and parallel circuits and dividers
Keep handy
Also worth your time
Why 3.3 V and 5 V parts need care, and how level shifting solves it