Using an expression to make something blink in After Effects is much more efficient than setting a bunch of opacity keyframes, especially if you have to make adjustments.

In this short tutorial video, I show you an expression to use to make something blink repeatedly. Although there are many ways to approach writing an expression for this task, I prefer using the Math.sin equation and creating a speed rate value for easy adjustment.

First, create a name for your speed rate and give it a value. Then, create the Math.sin equation, which will return a value between 1 and -1. Finally, write an if/else statement to tell After Effects what to do when the returned value is less than zero (in this case, make the opacity value 0) and what do to when the returned value is greater than 0 (in this case, make the opacity value 100). See below:

if(n<0) 0;
else 100;