Find all your DIY electronics in the MakerShed. 3D Printing, Kits, Arduino, Raspberry Pi, Books & more!
youritronicsPCB.jpg

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

Gareth Branwyn is a freelancer writer and the former Editorial Director of Maker Media. He is the author or editor of a dozen books on technology, DIY, and geek culture, including the first book about the web (Mosaic Quick Tour) and the Absolute Beginner’s Guide to Building Robots. He is currently working on a best-of collection of his writing, called Borg Like Me.


Related
blog comments powered by Disqus

Featured Products from the MakerShed

Follow

Get every new post delivered to your Inbox.

Join 25,442 other followers