Computer Mouse: 'An input device you least bother to learn about'

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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.

Inside 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: 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.

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:

Trackball Mouse

The rollers that touch the ball and detect X and Y motion.

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.

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 rate with which 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.

An onboard 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 send to the computer. You can also see the two buttons that detect clicks (on either side of the wire connector).

In this opto-mechanical 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 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:

Trackball MouseA close-up of one of the optical encoders that tracks 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 the other side, one half the height of one 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 time 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.

Most mice on the market today use a USB connector to connect to computer. USB is a standard way to connect all kinds of peripherals to computer, including printers, digital cameras, keyboards and mice.

Some older mice, many of which are still in use today, have a PS/2 connector, a few other older mice use a serial type of connector.

Developed by Agilent Technologies 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 LED's 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.

Optical Mouse Infrared LED and Sensor

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 trackball mice:

No moving parts means less wear and 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.

It's said that they don't require a special surface, such as a mouse pad.

Wireless Mice

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, a Bluetooth, or a built-in component.

Many electronics 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 MHz garage door openers operate at a frequency of 40 MHz, and 802.11b/g wireless networks operate at 2.4 GHz. 1 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,".

Unlike infrared technology, which is commonly used for short-range wireless communications such as television remote controls, RF devices don't 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 trackball system. Optical technology improves accuracy and lets you use the wireless mouse on almost any surface.

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 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 ot interference from other sources and RF devices.

Paring 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 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.

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