Arduino PWM output cannot drive MOSFET 0-12V On an Arduino Uno, the PWM pin 3 is connected to the gate pin of an N-channel MOSFET STP16NF06L. The source pin is connected to ground of a 12 V power supply and drain pin connected to a multimeter. The other end of the multimeter is connected to the positive terminal of the power supply.

Question: When Arduino varies its PWM output from 0 to 255, the multimeter reading goes from 11.50 V to 12.10 V. Why isn't it 0 to 12 V?

When the FET gate is connected to the Arduino pin that outputs either 0 V or 5 V, the multimeter reading is as expected at 0 V and 12 V respectively.
Assuming you have a suitable FET You should have a circuit in line the following.
http://elinux.org/RPi_GPIO_Interface_Circuits#Using_a_FET

You should have some kind of load (a multimeter will have too high an impedance). Use a resistor 4.7k or smaller. You should also have a resistor on the gate