Put 2 resistors as voltage divider, +5V - R1 - R2 - Ground. Connect arduino analog input to middle point R1 and R2. Than take 3-rd resistor and connect to same middle point. Input is other end R3. Voltage divider form by R3 and (R1||R2). Example: R1 = R2 = 2k. So (R1||R2) = 1k. Picking 9.1k as R3 and you have 10:1 divider. Now when input goes negative, arduino reads less than 2.5V, nothing connected = 2.5V and positive +8 - more than 2.5V Do a math.