Simple way to read analog sensor
There is no need of an arduino in this case. You can do it with few transistors and one opamp to compare sensor's output to "x".

Arduino or µC are use to do complexe operation, read bus, compute some data... Yes it's possible to do it with an arduino but as you said, it's overkill. 

If you look closer at the cost of your circuit, the design with arduino will be a way more expensive than the opamp (&lt;1$)