Apply Flicker to Pre-Composed Layer

[swshmn]

I have a pre-composed layer of seven video objects that I'd like to apply a flickering effect to. I want to be able to start and stop this flicker effect as I deem fit. 
I've tried applying a LoopOut expression to the opacity of this pre-composed layer in my main composition but it doesn't seem to work. 

When I try applying it to each individual video object inside the precomposition, I cannot control the length of the expression.

I've also tried creating a null object, and parenting this pre-composed layer, and then applying the LoopOut expression, but to no luck.

Any suggestions? 

[Towerguy]

Hi. Have a look at the Strobe Effect, it can be keyframed.

[Andrew Yoole]

"Flicker" can mean numerous things.  If you wish to simply flicker (vary) the opacity, as your first attempt suggests, then apply a Wiggle expression to the Precomp layer's opacity.  

f = 1;

a = 100;

wiggle(f, a)

f is for frequency, a is for amount.  This will create a random level of opacity at the frequency you set (above it's 1 per second) and the amount it varies (100 percent in this example).  The opacity will vary between each iteration, meaning if you set the frequency to 1 per second, it will slide from value to value each second.

To create a more clunky opacity change, where the opacity doesn't change between iterations, posterise time in the expression.

f = 1;

a = 100;

posterizeTime(f);

wiggle(f, a)

If you feel like spending a few dollars, videocopilot.net has a plugin called Twitch which allows lots of different ways to flicker, distort and twitch a layer, without ever thinking about expressions.