I wish more people knew that light curtain sensors are cheaply available and easy to integrate into an e-stop for automated machinery. you can protect a 3.0m by 0.5m region against ingress for under 70€.
if you're building hobbyist CNC stuff (milling, XY tables, robot arms, etc.) without a full-coverage interlocked enclosure they're a very affordable way to save you from serious injury.
