Exploding or alcoholic soda Well, I tried to follow a very simple recipe from "Cooking for Geeks" for Ginger Lemon Soda. 

I made a simple ginger syrup by cooking ~200g of chopped ginger together with 2 cups of sugar, then I used half of that syrup together with 1/2 a teaspoon of yeast (from the local home-brew store), and topped off with 900 ml of water.

The recipe says: Let rest at room temperature for two days, then refrigerate and drink.

Well, for the first batch, I had so much pressure that when I opened it, it came out like champagne and I lost half of the soda. 

For the second batch, I put it in the fridge after one day. Now the level of carbonation is just right, but it's still a bit alcoholic. I had half a glass and I can definitely tell that there was some alcohol involved.

I wonder: What is the way to go to get nice sparkling soda that isn't also high in alcohol?
This recipe is listed under the section for fermentation, together with beer, wine and mead. The section starts with the sentence "Wine, beer and traditional sodas all depend on yeast to ferment sugar into alcohol and generate carbonation". 

I don't know enough about the history of soda to know if early sodas were alcoholic. Or rather, I am quite sure that there were alcoholic, fermented, carbonated drinks long before what we call "soda" today existed, but I don't know if they were called soda. 

Whatever the language problem is, this recipe is definitely intended to produce a low-alcohol beverage, comparable to beer. If you want carbonated syrup, you should buy a carbonating machine. These take a bullet full of CO2 and press it into the drink base you have selected. 

As for the too-strong carbonation, this is probably due to the vague term "room temperature". Yeast growth speed depends on temperature. Because it is an exponential growth, even small changes in temperature can lead to vastly different results. If you want to repeat the experiment despite the alcohol production, try better controlling for the temperature. As I don't brew, I can't tell you the temperature for optimal carbonation after two days, you will have to find it out by yourself.