I've bought these special upright USB Type A ports with a "Sensor Contact" (the stray pin I've marked "Sensor Contact Pin"). I've been Googling on how they work, as well as sent about a half dozen emails to the tech support guy at the company (FCI) they come from asking the same question(s) like "How does it work?" and "Do you have examples?", and gotten nothing back but "They have the same pinout as other USB A ports", but not any relevant, helpful answers.

So that's why I'm turning to you guys. My assumption is that when a device is plugged in, it provide +5V through the "sensor" contact, which I could hook to an MCU or transistor or something.

What do you guys think?

Here's the Mouser page

Front Face - USB Type A with Sensor Contact Pins - USB Type A with Sensor Contact

  • \$\begingroup\$ Did you cut that out or? \$\endgroup\$
    – Passerby
    Feb 6, 2016 at 21:04
  • \$\begingroup\$ I got a feeling it's a simple switch. Check for continuity between the sensor pin and the pin that's below it, which should be pin 4, Ground. Then plug a cable in and see if continuity still exists or not. \$\endgroup\$
    – Passerby
    Feb 6, 2016 at 21:10
  • \$\begingroup\$ it looks to me like it's just an extra contact on the socket, and would require a plug that has a matching contact to be of any use, \$\endgroup\$ Feb 6, 2016 at 21:19
  • \$\begingroup\$ @Jasen A standard plug could deflect the GND contact such that touched the sense contact. \$\endgroup\$ Feb 6, 2016 at 21:24
  • \$\begingroup\$ Nick is correct \$\endgroup\$ Feb 8, 2016 at 21:17

2 Answers 2


[ I haven't come across a USB jack with a cable detection mechanism, until now.
So, the following is just a hypothesis. ]

This additional pin provides a mechanism for detecting if a cable is plugged into the jack, even if there is nothing at the end of the cable. Have a look at the pinout for the standard USB type-A jack. Notice that the sensor pin sits just above the standard GND pin. When a cable is plugged into the jack, it flexes the GND contact which touches the sensor pin. Essentially, it's a switch.

You can test easily this hypothesis.

  1. Check the continuity between the sense pin and the GND with a multimeter.
    It should be an open circuit.
  2. Then plug in a USB cable and check the continuity between GND and the sense pin again.
    Now it should be a closed circuit.

enter image description here

standard pinout of a USB type-A jack

  • \$\begingroup\$ sounds plausible, and the drawings from FCI support this hypothesis portal.fciconnect.com/Comergent/fci/drawing/73725.pdf \$\endgroup\$ Feb 6, 2016 at 21:33
  • \$\begingroup\$ Thank you! I was close, but had my contacts mixed up and thought that was +5V. But this is what I thought it was; a way to detect if something was plugged in. So the can I just hook that up to an MCU for a way to detect when things are plugged in? \$\endgroup\$ Feb 6, 2016 at 22:50
  • 1
    \$\begingroup\$ @Dominic You can treat it like any other mechanical switch. You can read it with a microcontroller (you'll need a pull-up resistor, obviously). \$\endgroup\$ Feb 6, 2016 at 22:55
  • \$\begingroup\$ Okay Just for double checking sake (not that I don't know how. I just want to be 100% sure I'm doing it correctly with this USB port), I just connect that sensor contact to an AVR pin, and connect that AVR pin to +5V through a 1MΩ resistor, correct? \$\endgroup\$ Feb 6, 2016 at 23:42
  • 1
    \$\begingroup\$ @Dominic You are mostly correct. 10kΩ (give or take) is a more common value for a pull-up resistor for a switch. \$\endgroup\$ Feb 6, 2016 at 23:53

The USB socket pins out the same as any other upright "flag" style. The sensor pin does indeed go to GND when a USB plug is inserted. Here are my pinout diagrams as well as the EAGLE FCI library with the included USB port in question. Hope this helps all ya'll out!

USB Type A Upright Flag Connector with Sense Contact 1 USB Type A Upright Flag Connector with Sense Contact 2

  • \$\begingroup\$ Hey @Dominic Luciano are you sure that your second picture here is correct? According to the datasheet the forward pin is called pin 1 (labelled gnd in bottom picture), while the leftmost pin in your top picture (+5V) is labelled "pos#1" in the datasheet. One would think that "pos#1" should correspond to "pin 1", but if that were true then your bottom picture would have its labels reversed. If that is the case then in reality this USB connector would connect the sensor contact pin to +5V when inserted \$\endgroup\$ Jun 21, 2017 at 3:26
  • \$\begingroup\$ Nope, it connects to GND, but thank you for the double check. \$\endgroup\$ Jun 27, 2017 at 14:49

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