Interrupt Arduino if voltage is below some level or a switch which will not allow current to pass across if no sufficient voltage
Given you are trying to detect line voltage, your level of electronics knowledge, and that this will be connected to a arduino, you really need to use isolation of some sort.

Here is a example:



The diodes full-wave rectify the AC line.  At your threshold of 180 VAC, the rectified DC will be 253 V.

The DC voltage is divided down by R1 and R2.  The voltage drop of the LED and the B-E junction of the transistor are used as the reference voltage.  These are rough and temperature-dependent, but it doesn't sound like you need high accuracy.  When the divided voltage gets to about 1.8 V, the LED in the opto is turned on.  R3 limits the LED current to a safe but sufficient level.

When the opto is turned on, it pulls low on OUT.  Otherwise, R4 pulls OUT high.  OUT being low therefore indicates the presence of AC roughly above the threshold.

Make sure the parts can withstand up to 400 V on the DC supply.  That leaves some margin above what should happen.  The 400 V needs to be considered when choosing the diodes, C1, R1, R3, and Q1.  It might be simpler to implement R1 and R3 as several smaller resistors in series.

At the threshold, the LED current is about 350 &micro;A.  The output of the opto needs to sink 330 &micro;A.  That means you need the opto to have a CTR (current transfer ratio) of about 1.  The opto shown has a minimum guaranteed CTR of 3, although at higher LED current.  Nonetheless, there is sufficient margin that this should work reliably.

Remember that everything on the left side of the opto is at dangerous voltages.  This section must be well insulated and sealed so nothing can accidentally touch it.