First off that will feed -5V into the arduino which will damage it.
but i mesured with multimeter it show 5V. i set VR to 14kOhm. the out put to arduino pin is 5V Grumpy_Mike,
Next as said the peak voltage for a 12V AC signal is not 12V but 12 times 1.4
what u mean it by 12 time 1.4? is the output will be 16.8V. it not verry clear to me.