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Can someone tell me the name and the function of the following component please?

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    \$\begingroup\$ wireless socket MMD \$\endgroup\$
    – greybeard
    Dec 29, 2023 at 5:30
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    \$\begingroup\$ Manufacturers: please remember that these are available in more colours than black and white, to make them easy to find on a PCB. \$\endgroup\$ Dec 29, 2023 at 12:57

4 Answers 4

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It is called a jumper. It's nothing more than a piece of metal (covered by some plastic on the outside) used to connect to adjacent pins.

From an electric point of view, it's the equivalent of a very short wire.

In practice, jumpers are mainly used to "configure" PCBs, for example :

  • choose if output voltage is 3.3V or 5V (the jumper will connect a pin connected to the output either to 3.3V or to 5V)
  • chose between always ON or controlled by an external switch (if the jumper is present, it connects 2 pins, and is therefore equivalent to a closed switch ; if you remove the jumper, you can use the switch to turn your device on and off)
  • activate or not a buzzer
  • set the "address" for some communication protocols (typical on I2C hobby boards)
  • on old PCs, indicate if a hard drive is a main drive or a secondary drive
  • on some computers (specially embedded ones), to have automatic turn on as soon as they are plugged in (otherwise you have to push a button as on a classical computer)
  • whatever other configuration the PCB designer wanted to provide (without getting into the trouble of making a full user interface)

There are also a few more technical uses :

  • on PCB prototypes : allow to "cut" a track (by removing the jumper) so that one can test part of the circuit independently
  • on PCB prototypes : allow to "cut" a track (by removing the jumper) so that one can insert an amperemeter to measure the current
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    \$\begingroup\$ "on all PCs, indicate if a hard drive is a main drive or a secondary drive" this seriously threw me back a couple of decades :D \$\endgroup\$ Dec 28, 2023 at 21:02
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    \$\begingroup\$ indeed, it is a typo : I meant on "old" (not "all") PCs, more specifically those with PATA/IDE hard drives. 10 years back, there where still plenty of those PCs around (not the new ones, but older ones, and most of the internal DVD players as far as I recall) \$\endgroup\$
    – Sandro
    Dec 28, 2023 at 22:52
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    \$\begingroup\$ (without getting into the trouble of making a full user interface) - Much more than just "trouble of making a UI" - jumpers allow configuration that is permanent (not subject to someone touching the wrong button or wrong thing on a touch-screen, etc.) and that is set even before any startup code has run. \$\endgroup\$ Dec 29, 2023 at 15:54
  • \$\begingroup\$ Good answer. I also (back in the day) had some experience with computer cards to add a serial port, and a jumper was used to determine what number the port would be. We would have COM1 fail on the motherboard and add COM2 using a card, because COM1 wasn't available any more. That might be covered by one of your uses. \$\endgroup\$
    – Wastrel
    Dec 29, 2023 at 17:27
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It is a jumper used to connect two adjacent terminals on a PCB. The spacing between the terminals can vary, but is usually 2.54 mm (0.1 inches).

The following is an example of configuring options on a development board, where all jumpers were connected except for the 5V supply connection between the JTAG debugger section (on the right) and the target microcontroller (on the left):

enter image description here

Where the 5V jumper was offset, so was not making an electrical connection, but was still on the board so I didn't misplace it if want to make the connection again.

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That's a jumper.

Used to connect or disconnect terminals on a PCB.

What the terminals do on a PCB is up to the circuitry and it is not useful to provide a list what can be done with jumpers.

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These are commonly known as Berg jumpers after the company that popularized them. They’re the 2 pin variety of the more generic Berg connector which can have more contacts, for example the power cable connector commonly used on older floppy drives.

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