Instead of a visible lever, use a "T-Flip Flop" circuit so that throwing an item into a specific corner triggers the wall.
Building a lovely craft piston trap is a relatively straightforward process that requires a basic understanding of Minecraft mechanics. Here's a step-by-step guide to get you started: lovely craft piston trap wiki free
If you tell me what you're trying to catch, I can suggest the for that specific mob. Instead of a visible lever, use a "T-Flip
Safety & detection