Electronic – No current through AC optocoupler

Those "electricians screwdrivers" detect voltage (not current) and while it may appear that there is only one contact point, they rely on the human body having a path to ground to complete the circuit.

The Arduino's analog inputs measure voltages between 0V and +5V. To measure current you must convert it into a voltage that is within this range. The simplest way to get voltage from current is to pass it through a resistor, then the voltage across the resistor is determined by Ohm's Law (V=I*R). For example if your buzzer draws 1 Amp and you insert a 1 Ohm resistor between the buzzer and ground, you will get 1 Volt.

However there are a couple of possible issues. Firstly, the resistor will steal 1 Volt from your buzzer (so it will buzz even more quietly). You can use a smaller resistance to reduce voltage drop, but then the signal also gets smaller. If you use a 0.1 Ohm resistor the voltage drop should be minimal, but the reading you get will be only be 0.1V per Amp. If you just want to tell whether whether the buzzer is on or off then this should be plenty. If you want accurate reading of lower currents then the voltage may have to be amplified.

Secondly, the current is AC - so the voltage you get will be a sine wave that goes both positive and negative. Since the Arduino can only measure positive voltage it will only 'see' the positive half cycles. This may be fine for your application. However the analog input could be damaged by negative voltage, so you should add a protection circuit to prevent the voltage from going below ground.

In the circuit below, R2 and D1 prevent the analog input from going below -0.3V.

^{simulate this circuit – Schematic created using CircuitLab}

The Fairchild datasheet for your part shows some more details of what's going on inside:

You can see that the 47 kOhm resistors on the AC pins are mainly limiting the current being delivered to the diode bridge. Exactly what voltage is seen by the internal circuits connected to the diode bridge depends on the I-V characteristics of those devices.