Référence des expressions blabla

https://helpx.adobe.com/fr/after-effects/using/expression-language-reference.html#global_objects_attributes_and_methods_expression_reference

Change layer opacity every n-frame

val1 = 100;
val2 = 0;
n = 15;
f = timeToFrames(time);
s = Math.floor(f/n);
s%2 ? val2 : val1

and switch the last line to this for the second group:

s%2 ? val1 : val2

Change value every n-frame

n = 15;
f = thisComp.frameDuration * n;
//The duration in seconds of ’n’ frames in this composition

//Evaluate everything after this line every n frames
Math.floor(random(1, 360));

Expression à placer sur la propriété d’opacité

blinkSpeed = 10;  
n = Math.sin(time*blinkSpeed);

if (n<=0) 0 else value;

Expression circonscrite dans le temps

blinkSpeed = 500;  
n = Math.sin(time*blinkSpeed);  
if(time > 30 && time <30.25){  
  if (n<=0) 0 else 100;  
}  
else{  
  100;  
}