If I understand you correctly :- you have a signal source that you wish to act as an input to the arduino. The signal can vary from 0 up to 7 volts or maybe higher. You want a saferty circuit to stop the signal from exceeding the arduino input limit of 5 volts.
Is this interpretation correct ?
OK if so you need a simple divider to drop the 0 - 7volts down to 0 - 5 volts then a safety feature to ensure it never exceeds 5 volts.
Place 2 resistors in series made up of a 390ohm and a 1000ohm. The junction of the two is the input to the arduino. The free end of the 390 goes to your signal and the free end of the 1000 goes to the common ground (of both signal and arduino. Now connect a 5 volt zener between the arduino input and the signal ground.
The arduino input will now read 1000/1390 or 71.9% of the signal, which, if it is 7 volts will be 5 volts, (5.03 actually if all resistor values are exact - which they won't be) with 5ma being drawn from the source (7/1390)
In excess of 5 volts the zener will start to conduct so passing more current through the 390 ohm resistor and increasing the voltage drop across it. The current through the 1000 ohm resistor remains constant at 5ma.
This will work but be aware that the measurement is only linear up to a signal level of 7 volts, after which it saturates and remains constant until the signal once more drops below 7 volts.