There’s a spy movie – probably from the [James Bond] franchise – in which our hero is staying in a fancy hotel. It’s crawling with enemies, naturally, and eager to see if one has been snooping in his room while he’s out for martinis, he sticks a hair across the gap in the door. When he comes back and finds the hair missing, he knows the game is afoot.

This hotel safe intrusion detector is what [Q] might have thought up for such a job if he’d had access to PIC microcontrollers and SMD LEDs. [Andy]’s “LightSafer” is a silent alarm for hotel safes, drawers, closets, or even the refrigerator – anywhere where the transition from dark to light indicates an unwanted visit. It’s tiny – only 33 x 21 mm – and is powered by a CR2032 coin cell. A Broadcom APDS-9300 light sensor watches for openings while the PIC monitors a joystick control for the correct PIN entry. There’s no audible alarm; rather, an LED blinks to indicate an unauthorized intrusion and blinks once for every 15 minutes since the event.

LightSafer is simple but effective, with a clever UI that keeps the current draw low and the battery life long. [Andy] used a similar technique for this low-draw cat tracking collar that we featured a while back.