# Volt-free contact sensing?

**URL:** <https://forum.sierrawireless.com/t/volt-free-contact-sensing/1041>\
**Category:** Legacy FX/FXT series\
**Created:** [January 25, 2007, 3:27pm UTC](https://forum.sierrawireless.com/t/volt-free-contact-sensing/1041 "2007-01-25T15:27:10Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![awneil](https://avatars.discourse-cdn.com/v4/letter/a/49beb7/32.png) [@awneil](https://forum.sierrawireless.com/u/awneil)\
**Post date:** [January 25, 2007, 3:27pm UTC](https://forum.sierrawireless.com/t/volt-free-contact-sensing/1041/1 "2007-01-25T15:27:10Z")

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Can the two GPIOs on the Fastrack be used to sense a simple open/close contact?

ie, no externally applied voltage, just the contact connected between the two GPIOs

```auto
GPIO4 -------+
             |
             |
             \
              \ Contact
               \
             |
             |
GPIO5 -------+
```

Could I then set one GPIO as an output, the other as an input, and detect whether the contact is open or closed?

Would I need to drive the output High or Low?

This would rely upon a GPIO set for input giving a defined value when open-circuit, and the other GPIO driving it to the opposite state when the contact closes.  
Would this work?

---

<div class="post-metadata">

**Author:** ![optomation](https://avatars.discourse-cdn.com/v4/letter/o/8e8cbc/32.png) [@optomation](https://forum.sierrawireless.com/u/optomation)\
**Post date:** [January 29, 2007, 12:30am UTC](https://forum.sierrawireless.com/t/volt-free-contact-sensing/1041/2 "2007-01-29T00:30:56Z")

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I am doing exactly that since November.  
I did some experiments and end up with the following procedure.

- Configure GPIO5 as OUTPUT and GPIO4 as INPUT.
- Turn GPIO5 ON (this answers one of you questions)
- Open/Close the circuit and view result in GPIO4.  
When done this way, the experiment works.

Curiously, If I use GPIO4 and GPIO5 the way around, by some reason, the state of GPIO5 has nothing to do with the state of the field loop.