We’ve got some friends who have two sump pumps. One is a backup and sounds an alarm when it is switched on. But this only works as long as they’re home to hear it. [Felix Rusu] came up with a solution ...

[John Boxall] took a different route for a single-input combination lock. This unit uses a Ping ultrasonic range finder to input a four digit code. It’s a hardware upgrade, but uses the same basic ...