Please answer these questions:
- Exactly how is the signal generator connected to the Arduino?
- Is the signal generator's ground connected to the Arduino's ground?
- What is the voltage of the signal generators output?
- What is the power frequency at your location?
- I can't tell, but I think that you might have connected a resistor to pin 8. What was the value of the resistance? What was the other end of the resistor connected to?
I suspect that you haven't connected the signal generator's ground to the Arduino's ground. Wait! Before you make that connection, you need to be sure that the signal generator is delivering a voltage that is friendly to the Arduino. That means that the signal voltage - the one that isn't ground - doesn't go lower than ground, or higher than VCC. If you can't tell, read the manual for the signal generator.
It seems to be difficult to get more than a small amount of information at a time, and what little there is isn't clear. Can you be a bit more forthcoming?