Arduino Technology
Understanding pull-up and pull-down resistors

When I first got involved in digital electronics, it took me awhile to understand the concepts of pull-up and pull-down resistors and when to use up or down, and when to use internal (built in, programmable) resistors, and when to use actual, discrete component, resistors.

On Youritronics (a name that will always make me wince), new editor Laci runs through a simple demo of pull-up and pull-down resistors, using the ATmega8 protostack.

In digital electronics the engineers use the terms pull-up or pull-down resistor, this is a simple resistor usually between 10k and 100k, to define the input state when no signal source is connected, this way overriding the noise signal, common sense dictates that when you have potentially larger noise then a smaller resistor is needed, but don’t be careless about it, don’t place a 100Ohm resistor because your signal source must be able to “defeat” the pull-up(down) resistor. A rule of thumb is to use at least 10x larger pull-up(down) resistor then your signal source impedance.

The pull-up term is used when the resistor connects the pin to the + and pull-down when connects to the ground, but then which should you choose? Good question, in these days it doesn’t really mater, your application or the pcb design should dictate, the older generation digital IC’s had asymmetrical current sink/source capability, they could sink more current and because of that the pull-up resistor was more common.

ATmega8 protostack simple demo


Gareth Branwyn is a freelance writer and the former Editorial Director of Maker Media. He is the author or editor of over a dozen books on technology, DIY, and geek culture. He is currently a contributor to Boing Boing, Wink Books, and Wink Fun. And he has a new best-of writing collection and “lazy man’s memoir,” called Borg Like Me.

View more articles by Gareth Branwyn