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Circuit Patterns, Part 3: Pull-Up and Pull-Down Resistors

If a part of a circuit is disconnected, the voltage of that circuit isn’t necessarily zero; it can pick up static electricity

In this series, we have been talking about the importance of circuit patterns for both understanding circuit schematics that you might find on the web and building your own circuits. This series introduces some of the commonly used circuit patterns that are essential to electronics.

Part 1 covered the importance of patterns and the most basic resistor pattern, the current limiting resistor. Part 2 covered voltage dividers, which—once you recognize them — you see in circuits everywhere. Part 3 covers two other important resistor patterns: pull-up and pull-down resistors.

To understand the importance of pull-up and pull-down resistors, you must first realize that, if a part of a circuit is disconnected, the voltage of that circuit isn’t necessarily zero. In fact, it is unknown. This is especially true of inputs to microcontrollers such as an Arduino or a single-board computer such as Raspberry Pi. These inputs use so little current that, if they are disconnected from any voltage, they can react to voltage fluctuations in the air as if they were changes to their inputs.

For instance, let’s say that you wanted a microcontroller to sense whether or not a button is pushed. It is tempting for new electronics hobbyists to simply connect a voltage source to an on/off button which goes to a pin on the microcontroller. This works fine while the button is pressed.

What happens when the hobbyist lets go of the button?

Letting go disconnects the microcontroller pin from the circuit. This does not mean that the voltage is zero, it means that the voltage is floating! Static electricity in the air will influence the voltage. All sorts of things that you can’t control will influence the voltage on that pin.

Therefore, we must make sure that the pin is always attached to a fixed voltage. We want the high, positive voltage when the button is pressed and a zero voltage when the button is released. It might be tempting to simply connect the pin to ground (zero voltage) directly, thinking that, when the button is pushed, you will get the voltage from the button and when it is not pushed, it will then drop to zero. The problem is that electricity always finds the easiest path to ground. So, when the button is pushed, it will skip the microcontroller pin entirely and just short out directly to ground.

To fix this situation, what is needed is a pull-down resistor. A pull-down resistor will keep the voltage high when the button is pushed but then pull the voltage low when the button is released. The size of the resistor will affect the operation as well. A larger resistor will waste less current when the button is pushed but it takes longer to stabilize the circuit when the button is let go. A smaller resistor will stabilize the circuit more quickly but waste more electricity while the button is pushed. In practice, for beginners, pretty much any resistor of 1,000 ohms or higher will work fine. In fact, just using the largest resistor you have is probably the best choice:

A pull-up resistor is just like a pull-down resistor but with a different “default” value. Pull-down resistors essentially say, “if this circuit gets disconnected, set the voltage to zero.” Pull-up resistors essentially say, “if this circuit gets disconnected, set the voltage to be the battery (or source) voltage.” Pull-down resistors get connected to ground and pull-up resistors get connected to a voltage source.

Pull-up resistors would be used if, for instance, the button itself was connected to ground instead of to the positive voltage source.

In practice, some parts of circuits may look like they are connected when, in reality, they aren’t.

Some input/output pins from transistors, microcontrollers, or integrated circuits may feature situations where the pin acts as if it were completely disconnected. In those cases, you need a pull-up or pull-down resistor to tell the circuit what its voltage is.

For more information on pull-up and pull-down resistors and other basic circuit patterns, have a look at my new book, Electronics for Beginners: A Practical Introduction to Schematics, Circuits, and Microcontrollers, published by technology publisher Apress (a Springer Nature company).

Here are Parts 1 and 2:

Circuit Patterns, Part I: Understanding circuit schematics You will get on much better in electronics if you learn to see the schematic line drawings as a series of patterns. When you begin to see the drawings in books on electronics as a connected series of familiar patterns, the world of electronics opens up.

Circuit Patterns, Part 2: Voltage Dividers When you see two resistors connected in series with a wire coming out from between them, the wire is likely a voltage divider. Knowing about voltage dividers will not only help you with projects, it will help you recognize this pattern on schematics you might find on the internet.

You may also want to have a look at:

New electronics book honors citizen scientist Forrest Mims III
Jonathan Bartlett’s dedication reflects Mims’ immense influence on electronics enthusiasts—including himself, as a boy. Electronics for Beginners follows in Mims’ footsteps as it shows the budding electronics enthusiast the many new components now available and how to use them.

Jonathan Bartlett

Senior Fellow, Walter Bradley Center for Natural & Artificial Intelligence
Jonathan Bartlett is a senior software R&D engineer at Specialized Bicycle Components, where he focuses on solving problems that span multiple software teams. Previously he was a senior developer at ITX, where he developed applications for companies across the US. He also offers his time as the Director of The Blyth Institute, focusing on the interplay between mathematics, philosophy, engineering, and science. Jonathan is the author of several textbooks and edited volumes which have been used by universities as diverse as Princeton and DeVry.

Circuit Patterns, Part 3: Pull-Up and Pull-Down Resistors