Wiring a button: pull-down bad vs INPUT_PULLUP best

[DaveX]

The Arduino button tutorial at:

How to Wire and Program a Button | Arduino Documentation

... suggests wiring the button as a pull-down, which could short the power to ground through the input pin if someone happened to run code with a pinMode(2,OUTPUT)

I've seen this diagram several times, but I've lost track of the source:

Can anyone point me to the source?

[LarryD]

The perpetrator might be me .

[xfpd]

This link does an image lookup.

[LarryD]

Share tips you have come across

Different circuit schematics to refer to: DIY basic Logic Probe and Logic Plulser. @ 5V, Logic Probe's load is ~1.6mA. [2025-06-27_00-57-58] [Switches and LEDs] let’s say we have circuit S1. Let’s say the switch is 10 feet from the Arduino. This means we are taking the +5v supply line out 10 feet and 10 feet back. 5v is now extended 20 feet with the potential of it coming in contact with things that can go pop if there was ever a short circuit on the cable. Also, sending 5v out on…

[DVDdoug]

@DaveX

.. suggests wiring the button as a pull-down, which could short the power to ground through the input pin if someone happened to run code with a pinMode(2,OUTPUT)

No, the resistor prevents a short to ground but you an get a short to 5V, which is just as bad. With a pull-up resistor, you can get a short to ground. The moral is - Don't mix-up inputs & outputs!

The advantage to pull-ups is that there is a built-in pull-up that you can enable.

[LarryD]

Share tips you have come across

Example: let’s say we have circuit S1. Let’s say the switch is 3 feet from the Arduino. This means we are taking the +5v supply line out 3 feet and 3 feet back. 5v is now extended 6 feet with the potential of it coming in contact with things that can go pop if there was ever a short circuit on the cable. If we are using S2, the pull-up is back at the Arduino, the switch is 3 feet away as before. If there was a short on the 6 feet of wire, the external pull-up limits the short circuit curre…

[DaveX]

Excellent.

That Share tips you have come across - #999 by LarryD post has the exact schematic image I was looking for.

I built one in a Wokwi at:

PullupPulldown - Wokwi ESP32, STM32, Arduino Simulator

[DaveX]

@DVDdoug

No, the resistor prevents a short to ground but you an get a short to 5V, which is just as bad.

The external resistor does not prevent shorting 5V to ground directly thorugh a digital LOW pin, bypassing the resistor.

I agree it is indeed bad if you mix up the inputs and outputs, but if the only source of 5V is the pinMode(pin,OUTPUT) and you short that to ground when you press the button, it might be somewhat less damaging that trying to sink the full power supply into the LOW output pin.

For a deployment, @LarryD's 5V on a long wire failure mode seems much more likely than my malicious code failure mode.

This article: 10 Ways to Destroy an Arduino — Rugged CircuitsRugged Industrial Arduino Microcontrollers has a good set of possible failure modes & solutions. Their trick is using a 30mA PTC on each output to limit current in out out of each pin.