Baking soda as a substitute for baking powder
Baking powder contains baking soda, plus acidic ingredient(s). 

If you have cream of tartar, you can make baking powder directly:


2 parts cream of tartar
1 part baking soda
1 part corn starch


Without cream of tartar, you can substitute baking soda for baking powder as long as you have an acid in your recipe, like buttermilk. If your recipe does not contain an acid, you should add one or the baking soda will remain inert. Based on the above proportions, you should be able to substitute 1/2 tsp baking soda + acid for 2 tsp baking powder.

For pancakes specifically, if you rest your batter, you should fold in the baking soda at the end so you do not lose all the leavening action.