Arduino digital input - Read voltage slightly higher then 5V - a power efficient solution
The 7V signal will go right to the Arduino through the capacitor and will create overvoltage on the micro. When the signal goes down to 0, after the cap has charged, it will drive the input below 0V, another no-no. 

A better way might be to use a MOSFET with the gate connected to the input. 







simulate this circuit &ndash; Schematic created using CircuitLab

The circuit will typically draw well under 1uA from the 0-7V circuit. R2 is just there to allow the gate protection zeners in Q1 to kill any spikes that much exceed +/-10v.