timropp:
They're switching full 120V AC and sending that as signals over to the electrical panel that actually runs the motor.So, is there an easy, cheap, safe way to detect button presses on something like this?
I AM NOT A QUALIFIED LICENSED ELECTRICIAN! DO NOT TAKE ANYTHING I SAY AS ANYTHING MORE THAN 'PSEUDOCIRCUIT!'
Pseudocircuit, like pseudocode, meaning an outline of how to do something, not the actual specific parts to use. This is the outline but there are 50 other safety and reliability aspects that need to be taken into account. For example, you don't want to tap into the system in a way that could -break- your lovely vintage mixer is something fails.
You need to detect whether a wire has 120V on it or not. Tap into it with a tiny 120V-6V transformer. Rectify that to make DC. Plug that into an optocoupler, and send the output from that to the Arduino.
Maybe a 7/8/9V transformer, because there will be losses losses during rectify. That's why you need an actual Electrician to design this.
There are probably black box module style off the shelf parts that can do this, with mains in and TTL out. Still need an Electrician to pick the right module for your exact situation because the integrity of your mixer is your ultimate responsibility.