Sinking current - How to pull down a point to zero using Arduino pins

The solution here screams "open collector"

Of course all of us with 20 years or more electronics experience know what you mean and have experienced that feeling but I don't think the OP appreciates that comment as much as the rest of us. A low tech option for people who should never be seen with a soldering iron in their hand (because of the disaster that would result: see photo)

is a small arduino friendly 5V relay board that can be easily interfaced with jumpers. They come in the 4-relay variety.
4-relay module