Run a macro based on changing Cell Value

Hi,
I have a formula where when I type a certain value in a cell (A1), another cell changes (A2). I then have a macro that reads the value in A2 and searches the database for a match of that cell and generates the appropriate image. The problem is that each time I do this, after typing a value into A1, I have to manually click the "Generate Drawing" button that I've made.

I'd like to figure out a way for excel to automatically run the macro as soon as I've entered my value into A1 instead of having to click "Generate Drawing" each time. If somebody could help me with this problem it would be greatly appreciated.