Question: I am looking at establish a circuit allowing to detect when one or two wire are cut such as a tamper system. When the wire are cut it should signal this a GPIO.
I suppose I need to use a transistor connected to GPIO but I have a bit some difficulty to see how.

Answer: 
![schematic](https://i.stack.imgur.com/A8UqT.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fA8UqT.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

*Figure 1. GPIO normally pulled low by tamper loop. Cutting loop causes GPIO to be pulled high by R1.*

C1 will help eliminate any noise on a long tamper loop.
Pull up the GPIO externally to HIGH. Configure a cheap uC to trigger an interrupt when the pin value changes from HIGH->LOW.When you enter the ISR,you'l know that your circuit was tampered/cut.

If you want to go a little further,you could use an [Event Monitor](http://www.nxp.com/documents/data_sheet/PCF2127.pdf) to capture such an event for you and log the time when it occurred.

Note: The value of the wire must remain at a known value and not change/toggle. That is always HIGH. Or if you need to detect a LOW->HIGH,you can configure appropriately.