How Computer Mice Work

This Microsoft Intellimouse uses optical technology. See more computer hardware pictures.

Mice first broke onto the public stage with the introduction of the Apple Macintosh in 1984, and since then they have helped to completely redefine the way we use computers.

Every day of your computing life, you reach out for your mouse whenever you want to move your cursor or activate something. Your mouse senses your motion and your clicks and sends them to the computer so it can respond appropriately.

In this article we'll take the cover off of this important part of the human-machine interface and see exactly what makes it tick.

Evolution of the Computer Mouse

It is amazing how simple and effective a mouse is, and it is also amazing how long it took mice to become a part of everyday life. Given that people naturally point at things -- usually before they speak -- it is surprising that it took so long for a good pointing device to develop. Although originally conceived in the 1960s, a couple of decades passed before mice became mainstream.

In the beginning, there was no need to point because computers used crude interfaces like teletype machines or punch cards for data entry. The early text terminals did nothing more than emulate a teletype (using the screen to replace paper), so it was many years (well into the 1960s and early 1970s) before arrow keys were found on most terminals. Full screen editors were the first things to take real advantage of the cursor keys, and they offered humans the first way to point.

Light pens were used on a variety of machines as a pointing device for many years, and graphics tablets, joy sticks and various other devices were also popular in the 1970s. None of these really took off as the pointing device of choice, however.

When the mouse hit the scene -- attached to the Mac, it was an immediate success. There is something about it that is completely natural. Compared to a graphics tablet, mice are extremely inexpensive and they take up very little desk space. In the PC world, mice took longer to gain ground, mainly because of a lack of support in the operating system. Once Windows 3.1 made Graphical User Interfaces (GUIs) a standard, the mouse became the PC-human interface of choice very quickly.

Inside a Mouse

The guts of a mouse
The guts of a mouse

The main goal of any mouse is to translate the motion of your hand into signals that the computer can use. Let's take a look inside a track-ball mouse to see how it works:

  1. A ball inside the mouse touches the desktop and rolls when the mouse moves. The underside of the mouse's logic board: The exposed portion of the ball touches the desktop.
  2. Two rollers inside the mouse touch the ball. One of the rollers is oriented so that it detects motion in the X direction, and the other is oriented 90 degrees to the first roller so it detects motion in the Y direction. When the ball rotates, one or both of these rollers rotate as well. The following image shows the two white rollers on this mouse: The rollers that touch the ball and detect X and Y motion
  3. The rollers each connect to a shaft, and the shaft spins a disk with holes in it. When a roller rolls, its shaft and disk spin. The following image shows the disk: A typical optical encoding disk: This disk has 36 holes around its outer edge.
  4. On either side of the disk there is an infrared LED and an infrared sensor. The holes in the disk break the beam of light coming from the LED so that the infrared sensor sees pulses of light. The rate of the pulsing is directly related to the speed of the mouse and the distance it travels. A close-up of one of the optical encoders that track mouse motion: There is an infrared LED (clear) on one side of the disk and an infrared sensor (red) on the other.
  5. An on-board processor chip reads the pulses from the infrared sensors and turns them into binary data that the computer can understand. The chip sends the binary data to the computer through the mouse's cord.
The logic section of a mouse is dominated by an encoder chip, a small processor that reads the pulses coming from the infrared sensors and turns them into bytes sent to the computer. You can also see the two buttons that detect clicks (on either side of the wire connector).

In this optomechanical arrangement, the disk moves mechanically, and an optical system counts pulses of light. On this mouse, the ball is 21 mm in diameter. The roller is 7 mm in diameter. The encoding disk has 36 holes. So if the mouse moves 25.4 mm (1 inch), the encoder chip detects 41 pulses of light.

You might have noticed that each encoder disk has two infrared LEDs and two infrared sensors, one on each side of the disk (so there are four LED/sensor pairs inside a mouse). This arrangement allows the processor to detect the disk's direction of rotation. There is a piece of plastic with a small, precisely located hole that sits between the encoder disk and each infrared sensor. It is visible in this photo:

A close-up of one of the optical encoders that track mouse motion: Note the piece of plastic between the infrared sensor (red) and the encoding disk.

This piece of plastic provides a window through which the infrared sensor can "see." The window on one side of the disk is located slightly higher than it is on the other -- one-half the height of one of the holes in the encoder disk, to be exact. That difference causes the two infrared sensors to see pulses of light at slightly different times. There are times when one of the sensors will see a pulse of light when the other does not, and vice versa. This page offers a nice explanation of how direction is determined.

Connecting Computer Mice

A typical PS/2 connector.
A typical PS/2 connector.

Most mice on the market today use a USB connector to attach to your computer. USB is a standard way to connect all kinds of peripherals to your computer, including printers, digital cameras, keyboards and mice. See How USB Ports Work for more information about this technology.

­ Some older mice, many of which are still in use today, have a PS/2 type connector. Instead of a PS/2 connector, a few other older mice use a serial type of connector to attach to a computer. See How Serial Ports Work for more information.


Optical Mice

In this photo, you can see the sensor on the bottom of the mouse.
In this photo, you can see the sensor on the bottom of the mouse.

­ Developed by Agilent Technologies and introduced to the world in late 1999, the optical mouse­ actually uses a tiny camera to take thousands of pictures every second.

Able to work on almost any surface without a mouse pad, most optical mice use a small, red light-emitting diode (LED) that bounces light off that surface onto a complimentary metal-oxide semiconductor (CMOS) sensor. In addition to LEDs, a recent innovation are laser-based optical mice that detect more surface details compared to LED technology. This results in the ability to use a laser-based optical mouse on even more surfaces than an LED mouse.

Here's how the sensor and other parts of an optical mouse work together:

  • The CMOS sensor sends each image to a digital signal processor (DSP) for analysis.
  • The DSP detects patterns in the images and examines how the patterns have moved since the previous image.
  • Based on the change in patterns over a sequence of images, the DSP determines how far the mouse has moved and sends the corresponding coordinates to the computer.
  • The computer moves the cursor on the screen based on the coordinates received from the mouse. This happens hundreds of times each second, making the cursor appear to move very smoothly.

Optical mice have several benefits over track-ball mice:

  • No moving parts means less wear and a lower chance of failure.
  • There's no way for dirt to get inside the mouse and interfere with the tracking sensors.
  • Increased tracking resolution means a smoother response.
  • They don't require a special surface, such as a mouse pad.

Optical Mouse Accuracy

A number of factors affect the accuracy of an optical mouse. One of the most important aspects is resolution. The resolution is the number of pixels per inch that the optical sensor and focusing lens "see" when you move the mouse. Resolution is expressed as dots per inch (dpi). The higher the resolution, the more sensitive the mouse is and the less you need to move it to obtain a respon­se.

Most mice have a resolution of 400 or 800 dpi. However, mice designed for playing electronic games can offer as much as 1600 dpi resolution. Some gaming mice also allow you to decrease the dpi on the fly to make the mouse less sensitive in situations when you need to make smaller, slower movements.

Historically, corded mice have been more responsive than wireless mice. This fact is changing, however, with the advent of improvements in wireless technologies and optical sensors. Other factors that affect quality include:

  • Size of the optical sensor -- larger is generally better, assuming the other mouse components can handle the larger size. Sizes range from 16 x 16 pixels to 30 x 30 pixels.
  • Refresh rate -- it is how often the sensor samples images as you move the mouse. Faster is generally better, assuming the other mouse components can process them. Rates range from 1500 to 6000 samples per second.
  • Image processing rate -- is a combination of the size of the optical sensor and the refresh rate. Again, faster is better and rates range from 0.486 to 5.8 megapixels per second.
  • Maximum speed -- is the maximum speed that you can move the mouse and obtain accurate tracking. Faster is better and rates range from 16 to 40 inches per second.

Wireless Mice

MX900 and docking station
MX900 and docking station
Photo courtesy Logitech

Most wireless mice use radio frequency (RF) technology to communicate information to your computer. Being radio-based, RF devices require two main components: a transmitter and a receiver. Here's how it works:

  • The transmitter is housed in the mouse. It sends an electromagnetic (radio) signal that encodes the information about the mouse's movements and the buttons you click.
  • The receiver, which is connected to your computer, accepts the signal, decodes it and passes it on to the mouse driver software and your computer's operating system.
  • The receiver can be a separate device that plugs into your computer, a special card that you place in an expansion slot, or a built-in component.

Many electronic devices use radio frequencies to communicate. Examples include cellular phones, wireless networks, and garage door openers. To communicate without conflicts, different types of devices have been assigned different frequencies. Newer cell phones use a frequency of 900 megahertz, garage door openers operate at a frequency of 40 megahertz, and 802.11b/g wireless networks operate at 2.4 gigahertz. Megahertz (MHz) means "one million cycles per second," so "900 megahertz" means that there are 900 million electromagnetic waves per second. Gigahertz (GHz) means "one billion cycles per second." To learn more about RF and frequencies, see How the Radio Spectrum Works.

Unlike infrared technology, which is commonly used for short-range wireless communications such as television remote controls, RF devices do not need a clear line of sight between the transmitter (mouse) and receiver. Just like other types of devices that use radio waves to communicate, a wireless mouse signal can pass through barriers such as a desk or your monitor.

RF technology provides a number of additional benefits for wireless mice. These include:

  • RF transmitters require low power and can run on batteries
  • RF components are inexpensive
  • RF components are light weight

As with most mice on the market today, wireless mice use optical sensor technology rather than the earlier track-ball system. Optical technology improves accuracy and lets you use the wireless mouse on almost any surface -- an important feature when you're not tied to your computer by a cord.

Pairing and Security

In order for the transmitter in the mouse to communicate with its receiver, they must be paired. This means that both devices are operating at the same frequency on the same channel using a common identification code. A channel is simply a specific frequency and code. The purpose of pairing is to filter out interference from other sources and RF devices.

Pairing methods vary, depending on the mouse manufacturer. Some devices come pre-paired. Others use methods such as a pairing sequence that occurs automatically, when you push specific buttons, or when you turn a dial on the receiver and/or mouse.

To protect the information your mouse transmits to the receiver, most wireless mice include an encryption scheme to encode data into an unreadable format. Some devices also use a frequency hopping method, which causes the mouse and receiver to automatically change frequencies using a predetermined pattern. This provides additional protection from interference and eavesdropping.

Bluetooth Mice

One of the RF technologies that wireless mice commonly use is Bluetooth. Bluetooth technology wirelessly connects peripherals such as printers, headsets, keyboards and mice to Bluetooth-enabled devices such as computers and personal digital assistants (PDAs). Because a Bluetooth receiver can accommodate multiple Bluetooth peripherals at one time, Bluetooth is also known as a personal area network (PAN). Bluetooth devices have a range of about 33 feet (10 meters).

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Bluetooth operates in the 2.4 GHz range using RF technology. It avoids interference among multiple Bluetooth peripherals through a technique called spread-spectrum frequency hopping. WiFi devices such as 802.11b/g wireless networks also operate in the 2.4 GHz range, as do some cordless telephonescordless telephones and microwave ovens. Version 1.2 of Bluetooth provides adaptive frequency hopping (AFH), which is an enhanced frequency-hopping technology designed to avoid interference with other 2.4 GHz communications.

RF Mice

The other common type of wireless mouse is an RF device that operates at 27 MHz and has a range of about 6 feet (2 meters). More recently, 2.4 GHz RF mice have hit the market with the advantage of a longer range -- about 33 feet (10 meters) and faster transmissions with less interference. Multiple RF mice in one room can result in cross-talk, which means that the receiver inadvertently picks up the transmissions from the wrong mouse. Pairing and multiple channels help to avoid this problem.

Typically, the RF receiver plugs into a USB port and does not accept any peripherals other than the mouse (and perhaps a keyboard, if sold with the mouse). Some portable models designed for use with notebook computers come with a compact receiver that can be stored in a slot inside the mouse when not in use.

Mouse Innovations

As with many computer-related devices, mice are being combined with other gadgets and technologies to create improved and multipurpose devices. Examples include multi-media mice, combination mice/remote controls, gaming mice, biometric mice, tilting wheel mice and motion-based mice. To learn more about innovations in mouse technology, let's start with multi-media mice and combination mice/remote controls.

­Multi-Media Mouse and Combination Mouse/Remote

These types of mice are used with multimedia systems such as the Windows XP Media Center Edition computers. Some combine features of a mouse with additional buttons (such as play, pause, forward, back and volume) for controlling media. Others resemble a television/media player remote control with added features for mousing. Remote controls generally use infrared sensors but some use a combination of infrared and RF technology for greater range.

Gaming Mice

­ Gaming mice are high-precision, optical mice designed for use with PCs and game controllers. Features may include:

  • Multiple buttons for added flexibility and functions such as adjusting dpi rates on the fly
  • Wireless connectivity and an optical sensor
  • Motion feedback and two-way communication

Motion-Based Mice

Yet another innovation in mouse technology is motion-based control. With this feature, you control the mouse pointer by waving the mouse in the air.

The technology patented by one manufacturer, Gyration, incorporates miniature gyroscopes to track the motion of the mouse as you wave it in the air. It uses an electromagnetic transducer and sensors to detect rotation in two axes at the same time. The mouse operates on the principle of the Coriolis Effect, which is the apparent turning of an object that's moving in relation to another rotating object. The device and accompanying software converts the mouse movements into movements on the computer's screen. The mice also include an optical sensor for use on a desktop.

Biometric Mice

The Wireless IntelliMouse Explorer with Fingerprint Reader is a biometric mouse.
The Wireless IntelliMouse Explorer with Fingerprint Reader is a biometric mouse.
Photo courtesy Microsoft Corporation

Biometric mice add security to your computer system by permitting only authorized users to control the mouse and access the computer. Protection is accomplished with an integrated fingerprint reader either in the receiver or the mouse. This feature enhances security and adds convenience because you can use your fingerprint rather than passwords for a secure login.

To use the biometric feature, a software program that comes with the mouse registers fingerprints and stores information about corresponding authorized users. Some software programs also let you encrypt and decrypt files. For more information about biometric fingerprint technology, see How Fingerprint Scanners Work.

Tilting Scroll Wheel

Logitech Cordless Click! Plus
Logitech Cordless Click! Plus
Photo courtesy Logitech

A more recent innovation in mouse scrolling is a tilting scroll wheel that allows you to scroll onscreen both horizontally (left/right) and vertically (up/down). The ability to scroll both ways is handy when you are viewing wide documents like a Web page or spreadsheet.

To navigate both horizontally and vertically, the scroll wheel is positioned on a combination fulcrum and lever. This is the design used by the Logitech Cordless Click! Plus mouse.

Another method for vertical and horizontal scrolling is a touch scroll panel that responds to your finger sliding horizontally and vertically, as employed by the Logitech V500 Cordless Notebook Mouse.

Logitech V500 Cordless Notebook Mouse
Photo courtesy Logitech

For more information on mice and related topics, check out the links on the next page.

Related Articles

More Great Links


  • Agilent Technologies.
  • Gyration.
  • Logitech.
  • Microsoft Corporation.
  • Optical Mouse Technology Review, by Richard L. Owens.