Additional quest re: popular vote for pres?

I'm well aware that the electoral college overrides popular vote for pres. What I'm asking is when there started to be a popular vote. The electors now vote the way the people in their state voted in the popular election. Per the constitution, they are each states electors are to pick 2 candidates (based on their own opinion) and their votes are counted by the Pres of the US Senate and the winner is the president. There is no mention of primaries or caucuses being used to determine the candidates, or of a popular vote determining who the electors vote for. I have checked Wikepedia, I have read the constitution and it seems we are electing a president our way with no justification in the constitution. According to Newsweek the founders set it up this way to make sure that a person who is a good speaker and charismatic cannot win the presidency based on popular opinion. Does it not seem that this is what is happening?